staging:iio:ad7793: Report channel offset

In bipolar mode there is a a binary offset of 2**(N-1) (with N being the number
of bits) on the reported value. Currently this value is subtracted when doing a
manual read. While this works for manual channel readings it does not work for
buffered mode. So report the offset in the channels offset property, which will
work in both modes.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>