staging: iio: adc: ad7606: Misc style fixes (no functional change)

* Placed includes in alphabetical order
* Added brackets around num and mask through out for AD760X_CHANNEL
* Used single line comments where needed
* Removed extra lines and spaces

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>