i2c: changing file names to be consistent

Naming convention uses the _ not - in the file names.  In a recent
commit for I2C this for some reason changed.  Making the names now
more consistent with other files.

Change-Id: I1e363c0c09ae0c8b2ac3def3beeaaeafcc01eb74
Signed-off-by: Dan Kalowsky <daniel.kalowsky@intel.com>

drivers/i2c/Makefile

@@ -1 +1 @@
-obj-$(CONFIG_I2C_DW) += i2c-dw.o i2c_static_irq_stubs.o
+obj-$(CONFIG_I2C_DW) += i2c_dw.o i2c_static_irq_stubs.o

drivers/i2c/i2c_dw.c

@@ -33,8 +33,8 @@
 #include <shared_irq.h>
 #endif
 
-#include "i2c-dw.h"
-#include "i2c-dw-registers.h"
+#include "i2c_dw.h"
+#include "i2c_dw_registers.h"
 
 #ifndef CONFIG_I2C_DEBUG
 #define DBG(...) {;}