iio: amplifiers: ad8366: rework driver to allow other chips

The SPI gain amplifiers are simple devices, with 1 or 2 channels, to which
are read-from/written-to.

The gain computation in ad8366_write_raw() has been updated to compute gain
in dB for negative values.

This driver will be extended to support other chips as well.
To do that, this rework handles the AD8366 device as a special-case (via
switch-statements). This will make things easier when adding new chips.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>