Staging: iio: adc: Refine delay computation

As per comment in function ad7280_delay, conversion delay may take up
to 250us. We cannot rely on msleep for this because it may sleep longer
than intended time for values less than 20ms, so use usleep_range instead.

Problem detected using checkpatch.pl.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>