staging: iio: adc: ad799x: Address iio list feedback

Address feedback from Shubhrajyoti Datta:
	Cast return value
Michael Hennerich:
	Use i2c_smbus_read_byte_data()
	Fix return value check

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>