Added a condition for a read of IIO_CHAN_INFO_OFFSET to ensure
that an offset is only provided for requests for XADC temperature.
The previous assumption was that only requests for temperature would
include a read of CHAN_INFO_OFFSET, but there is no such
restriction for iio_convert_raw_to_processed_unlocked() which is called
when using the XADC as a device for iio_hwmon.

Signed-off-by: Joshua Lamorie <joshua.lamorie@xiphos.com>