iio: adc: nau7802: convert probe to full device-managed

iio: adc: nau7802: convert probe to full device-managed

This is a trivial conversion to device-managed functions.
The mutex_destroy() calls are redundant, as the data will be free'd anyway.

And the IRQ and IIO register functions both have device-managed
equivalents.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210926154932.3287590-1-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>