Add proper mutex guards as we should not be able to disable
the core in the middle of enabling it.

Note there's no need to rush in backporting this as the only user of the
backend does not do anything crazy..

Fixes: 794ef0e5 ("iio: adc: adi-axi-adc: move to backend framework")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240531-dev-axi-adc-drp-v3-1-e3fa79447c67@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>