[MIPS] Use __ffs() instead of ffs() in ip32_irq0().

    
With recent rewrite for generic bitops, ffs() is defined the same way
as the libc and compiler built-in routines (returns int instead of
unsigned long).  Use __ffs() for 64bit value.
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>