asm-generic: fix ffs -Wshadow warning

gcc -Wshadow warns about the ffs() definition that has the
same name as the global ffs() built-in:

include/asm-generic/bitops/builtin-ffs.h:13:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]

This is annoying because 'make W=2' warns every time this
header gets included.

Change it to use a #define instead, making callers directly
reference the builtin.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/bitops/builtin-ffs.h

@@ -10,9 +10,6 @@
  * the libc and compiler builtin ffs routines, therefore
  * differs in spirit from the above ffz (man ffs).
  */
-static __always_inline int ffs(int x)
-{
-	return __builtin_ffs(x);
-}
+#define ffs(x) __builtin_ffs(x)
 
 #endif