staging:iio:adc:ad7280a: rename _read() to _read_reg()

This avoids possible confusion with read back of the channel conversions.
These two types of reads are of difference sizes with resulting differences
in the data layout of the response from the hardware.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-4-jic23@kernel.org