iio: accel: mma8452: Factor out guts of write_raw() to simplify locking

Factoring out those parts of write_raw() in which direct mode is held
allows for direct returns on errors, simplifying the code.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250217140135.896574-3-jic23@kernel.org


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