staging: iio: cdc: ad7746: Factor out ad7746_read_channel()

Reduce deep indenting and simplify the locking cleanup that follows.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-7-jic23@kernel.org