iio: adc: ad7091r-base: Use auto cleanup of locks.

Done to reduce boilerplate and simplify code flow by allowing early
returns with the lock automatically released.

Reviewed-by: Nuno Sa <nuno.a@analog.com>
Link: https://lore.kernel.org/r/20240128150537.44592-11-jic23@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>