Staging: iio: light: Use usleep_range instead of msleep

This patch fixes following checkpatch.pl warning:

WARNING: msleep < 20ms can sleep for up to 20ms; see Documentation/timers/timers-howto.txt

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>