staging: iio: cdc: ad7764: Push locking down into case statements in read/write_raw

Not all paths require any locking at all. So to simplify the
removal of such locking push the locks down into the individual
case statements.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-8-jic23@kernel.org