uart: make mraa_uart_get_dev_path return const char*

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

include/mraa_internal_types.h

@@ -91,7 +91,7 @@ struct _aio {
 struct _uart {
     /*@{*/
     int index; /**< the uart index, as known to the os. */
-    char* path; /**< the uart device path. */
+    const char* path; /**< the uart device path. */
     int fd; /**< file descriptor for device. */
     /*@}*/
 };