iio: amplifiers: ad8366: prepare for device-tree support

iio: amplifiers: ad8366: prepare for device-tree support

Drop switch case on the enum ID in favor of extended chip info table,
containing:
- gain_step, indicating with sign the start of the code range;
- num_channels, to indicate the number IIO channels;
- pack_code() function to describe how SPI buffer is populated;

Which allowed for a simplified read_raw() and write_raw() callbacks. The
probe() function was adjusted accordingly. The linux/array_size.h include
is removed as number of channels is provided by chip info table.

Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>