iio: adc: ad7192: Factor out core of ad7192_write_raw() to simplify error handling.

Factor out everything under the lock, use guard() for the mutex to
avoid need to manually unlock, and switch sense of matching checks
in loops to reduce indent.

There are some functional changes in here as well as the code
no longer updates the filter_freq_available if no change has
been made to the oversampling ratio.

Cc: Alisa-Dariana Roman <alisa.roman@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250217141630.897334-11-jic23@kernel.org


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