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

commit 04f5b9f5 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>