iio: adc: max1027: Add debugfs register read support

Until now, only write operations were supported. Force two bytes read
operation when reading, which should fit most of the development
purposes. Of course, extended operations like buffered reads on
multiple channels or even temperature + voltage reads will not be read
entirely. Usually, just starting a new operation will work but in any
case a software reset (done through the debufs interface too) will
return the device in a usable state.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>