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

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

BugLink: https://bugs.launchpad.net/bugs/1869061
commit a500f3bd787f8224341e44b238f318c407b10897 upstream.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>