iio: adc: at91-sama5d2_adc: fix differential channels in triggered mode

iio: adc: at91-sama5d2_adc: fix differential channels in triggered mode

[ Upstream commit a500f3bd787f8224341e44b238f318c407b10897 ]

The differential channels require writing the channel offset register (COR).
Otherwise they do not work in differential mode.
The configuration of COR is missing in triggered mode.

Fixes: 5e1a1da0f8c9 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>