iio: adc: ad7791: Factor out core of ad7791_write_raw() to simplify error handling

Factor out everything under the direct mode claim allowing direct returns
in error paths.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250217141630.897334-16-jic23@kernel.org


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