staging:iio:adc:ad7280a: Remove shift from cb_mask state cache.

Making the local storage of the Cell Balance mask a simple
bitmap and then shifting it only at time of register write simplifies
several code paths.

Suggested-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-19-jic23@kernel.org