staging: iio: adc: ad7280a: split ad7280_channel_init() to more functions

The ad7280_channel_init function has been split into more specific
functions to increase the code readability.

The setting of channel's scan_type.shift to 0, has been removed, since
it is the default value.

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>