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 */