An existing comment already mentions we: "cannot use devm_iio_channel_get
because x86 systems lack the device<->channel maps which iio_channel_get
will try to use when passed a non NULL device pointer".

Work around this by registering a devm action to free the iio-channels.
This is a step on the way to fully converting the probe() function to
only use devm managed resources.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>