iio: adc: ad7124: fix temperature channel

iio: adc: ad7124: fix temperature channel

Fix temperature channel not working due to gain and offset not being
initialized.  For channels other than the voltage ones calibration is
skipped (which is OK).  However that results in the calibration register
values tracked in st->channels[i].cfg all being zero.  These zeros are
later written to hardware before a measurement is made which caused the
raw temperature readings to be always 8388608 (0x800000).

To fix it, we just make sure the gain and offset values are set to the
default values and still return early without doing an internal
calibration.

While here, add a comment explaining why we don't bother calibrating
the temperature channel.

Fixes: 47036a03a303 ("iio: adc: ad7124: Implement internal calibration at probe time")
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>