iio: adc: mediatek: fix unset field

dev_comp field is used in a couple of places but it is never set. This
results in kernel oops when dereferencing a NULL pointer. Set the
`dev_comp` field correctly in the probe function.

Fixes: 6d97024d ("iio: adc: mediatek: mt6577-auxadc, add mt6765 support")
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201018194644.3366846-1-fparent@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>