m68k/bitops: convert __ffs to match generic declaration

The generic bitops declare __ffs as

	static inline unsigned long __ffs(unsigned long word);

Convert the m68k version to match the generic declaration.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>