iio: adc: mcp3911: simplify code with guard macro

Use the guard(mutex) macro for handle mutex lock/unlocks.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20231206-mcp3911-guard-v4-1-30c3c5d4340f@gmail.com


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