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>