iio: pressure: mpl3115: factor out core of IIO_INFO_RAW read to simplify code flow

iio: pressure: mpl3115: factor out core of IIO_INFO_RAW read to simplify code flow

Apply guard(mutex) to remove the need for manual release of the lock.
Factor out the code that occurs under the direct claim.
These two changes allow for direct returns simplifying code flow.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://patch.msgid.link/20250331121317.1694135-27-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>