drivers/rtc/rtc-ab8500.c: change msleep() to usleep_range()
The resolution of msleep is related to HZ, so with HZ set to 100 any
msleep of less than 10ms will become ~10ms. This is not what we want.
Use the hrtimer-based usleep_range() and allow for some slack in the
non-critical path so we have more control of what is happening here.