staging: iio: cdc: ad7746: avoid overwrite of num_channels

AD7745 devices don't have the CIN2 pins and therefore can't handle related
channels. Forcing the number of AD7746 channels may lead to enabling more
channels than what the hardware actually supports.
Avoid num_channels being overwritten after first assignment.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Fixes: 83e416f4 ("staging: iio: adc: Replace, rewrite ad7745 from scratch.")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: <Stable@vger.kernel.org>