staging:iio:adc:ad799x address and scan_index always match so stop using address

Saves on setting the value of address for the simple situation seen in this
device. They are already used interchangably to get data from the buffer.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>