iio: qcom-rradc: Check for USB presence

Check whether USB is connected while doing conversion for
channels that require USB to read valid readings. If USB
is not plugged in, exit loop and return error.

Change-Id: Ib181a9e4406d88dc2988ca08e84598635e5a4716
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>