mtd: spi-nor: use scope-based mutex cleanup helpers

Use scope-based mutex clenup helpers, it reduces the code size.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20250211-spi-nor-guard-mutex-v1-2-05ed77a484d9@linaro.org