ab8500_fg: Replace msleep() with usleep_range() for greater accuracy

Doing so provides a greater degree of accuracy when dealing with
time-frames between 1us and 20ms. msleep() is only accurate for
wake-ups greater than 20ms.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonas ABERG <jonas.aberg@stericsson.com>
Reviewed-by: Johan BJORNSTEDT <johan.bjornstedt@stericsson.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>