iio: adc: ad7124: use guard(mutex) to simplify return paths

iio: adc: ad7124: use guard(mutex) to simplify return paths

Use guard(mutex) in a couple of functions to allow direct returns. This
simplifies the code a bit and will make later changes easier.

cleanup.h was already included for prior use of __free()

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>