IIO: Change msleep to usleep_range for small msecs

msleep(1~20) may not do what the caller intends, and will often sleep longer.
(~20 ms actual sleep for any value given in the 1~20ms range)
This is not the desired behaviour for many cases like device resume time,
device suspend time, device enable time, data reading time, etc.
Thus, change msleep to usleep_range for precise wakeups.

Signed-off-by: Aniroop Mathur <a.mathur@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>