staging:iio:ad7887: Allow to use internal ref in two channel mode

While it is not recommended to use the internal reference in two channel mode in
order to obtain optimal performance it is still possible to use it.

While we are at it also get rid of the duplicate tx_cmd_buf entries. There are
only two unique entries. One for channel 1 and one for channel 2.

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