timer: optimize apply_slack()

__fls(mask) is equivalent to find_last_bit(&mask, BITS_PER_LONG), but cheaper.
find_last_bit was showing up high on the list when I was profiling for stalls
on icache misses on a system with very small cache size (MIPS).

Change-Id: Ia7e6632c2a4c2ef10bb7f7b7a86dcfc5373d2659
Signed-off-by: Felix Fietkau <nbd@openwrt.org>