drivers: rearrange for standard use of extern "C" in private headers

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Only updated in headers that already had support for drivers built with
a C++ compiler.

The spi_dw.h file defines macros to declare functions, then uses them
within a file that may have out-of-tree overrides.  In this case we
leave the including file extern "C" active for backward compatibility.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

drivers/audio/mpxxdtyy.h

@@ -7,15 +7,15 @@
 #ifndef MPXXDTYY_H
 #define MPXXDTYY_H
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <audio/dmic.h>
 #include <zephyr.h>
 #include <device.h>
 #include "OpenPDMFilter.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define MPXXDTYY_MIN_PDM_FREQ		1200000 /* 1.2MHz */
 #define MPXXDTYY_MAX_PDM_FREQ		3250000 /* 3.25MHz */
 

drivers/i2c/i2c-priv.h

@@ -7,14 +7,14 @@
 #ifndef ZEPHYR_DRIVERS_I2C_I2C_PRIV_H_
 #define ZEPHYR_DRIVERS_I2C_I2C_PRIV_H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <drivers/i2c.h>
 #include <dt-bindings/i2c/i2c.h>
 #include <logging/log.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 static inline u32_t i2c_map_dt_bitrate(u32_t bitrate)
 {
 	switch (bitrate) {

drivers/spi/spi_dw.h

@@ -9,8 +9,11 @@
 #ifndef ZEPHYR_DRIVERS_SPI_SPI_DW_H_
 #define ZEPHYR_DRIVERS_SPI_SPI_DW_H_
 
+#include <string.h>
 #include <drivers/spi.h>
 
+#include "spi_context.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -29,8 +32,6 @@ struct spi_dw_config {
 	u8_t op_modes;
 };
 
-#include "spi_context.h"
-
 struct spi_dw_data {
 #ifdef CONFIG_CLOCK_CONTROL
 	struct device *clock;
@@ -194,6 +195,10 @@ struct spi_dw_data {
 /*
  * Including the right register definition file
  * SoC SPECIFIC!
+ *
+ * The file included next uses the DEFINE_MM_REG macros above to
+ * declare functions.  In this situation we'll leave the containing
+ * extern "C" active in C++ compilations.
  */
 #include "spi_dw_regs.h"
 
@@ -215,8 +220,6 @@ DEFINE_TEST_BIT_OP(sr_busy, DW_SPI_REG_SR, DW_SPI_SR_BUSY_BIT)
 
 #ifdef CONFIG_CLOCK_CONTROL
 
-#include <string.h>
-
 static inline int clock_config(struct device *dev)
 {
 	const struct spi_dw_config *info = dev->config->config_info;
@@ -260,7 +263,8 @@ static inline void clock_off(struct device *dev)
 
 	extra_clock_off(dev);
 }
-#else
+
+#else /* CONFIG_CLOCK_CONTROL */
 #define clock_config(...)
 #define clock_on(...)
 #define clock_off(...)
@@ -269,4 +273,5 @@ static inline void clock_off(struct device *dev)
 #ifdef __cplusplus
 }
 #endif
+
 #endif /* ZEPHYR_DRIVERS_SPI_SPI_DW_H_ */