iio: adc: adi-axi-adc: split axi_adc_chan_status()

Add a new axi_adc_read_chan_status() helper so we get the raw register
value out of it.

This is in preparation of a future change where we really want to look
into dedicated bits of the register.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240802-dev-iio-backend-add-debugfs-v2-5-4cb62852f0d0@analog.com


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