[MIPS] Alchemy: replace ffs() with __ffs()

Fix havoc wrought by commit 56f621c7f6f735311eed3f36858b402013023c18 --
au_ffs() and ffs() are equivalent, that patch should have just replaced one
with another.  Now replace ffs() with __ffs() which returns an unbiased bit
number.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>