irqchip/mips-gic: Use for_each_set_bit to iterate over IRQs

The MIPS GIC driver has previously iterated over bits set in a bitmap
representing pending IRQs by calling find_first_bit, clearing that bit
then calling find_first_bit again until all bits are clear. If multiple
interrupts are pending then this is wasteful, as find_first_bit will
have to loop over the whole bitmap from the start. Use the
for_each_set_bit macro which performs exactly what we need here instead.
It will use find_next_bit and thus only scan over the relevant part of
the bitmap, and it makes the intent of the code more clear.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Link: https://lkml.kernel.org/r/20160819171119.28121-1-paul.burton@imgtec.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>