Fix build on PowerPC64

On PowerPC64 architecture get_ra() is also present in Linux but with a
different name causing build to fail. To fix this let's rename local
get_ra() to rtl_get_ra().

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>

include/wifi.h

@@ -463,7 +463,7 @@ __inline static int IS_MCAST(const u8 *da)
 		return _FALSE;
 }
 
-__inline static unsigned char *get_ra(unsigned char *pframe)
+__inline static unsigned char *rtl_get_ra(unsigned char *pframe)
 {
 	unsigned char	*ra;
 	ra = GetAddr1Ptr(pframe);