ASoC: ab8500: add range to usleep_range

Providing a range for usleep_range() allows the hrtimer subsystem to
coalesce timers - as this delay has no upper limit anyway (interrupts
or context switch is possible) it should not hurt to extend this
from 2 to 2-4 milliseconds.

Signed-off-by: Nicholas Mc Guire <hofrat@opentech.at>
Signed-off-by: Mark Brown <broonie@kernel.org>