MIPS: TXx9: Microoptimize interrupt handlers

The IOC interrupt status register on RBTX49XX only have 8 bits.  Use
8-bit version of __fls() to optimize interrupt handlers.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>