Staging: iio: accel: Replace msleep with usleep for given range

This patch fixes checkpatch.pl warning:
WARNING: msleep < 20ms can sleep for up to 20ms; see Documentation/timers/timers-howto.txt

As per Documentation/timers/timers-howto.txt usleep_range should be used
for sleeping for 10us-20ms

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>