iio: adc: max11410: Factor out writing of sampling frequency to simplify errro paths.

Introduce __max11410_write_samp_freq() helper and use guard() to avoid
need for manual unlock of the mutex. This allows direct returns and
simplifies the resulting error handling.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250217141630.897334-26-jic23@kernel.org


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