staging:iio:adc:ad7291: fix channel mapping

ideally in_voltageX_raw should map to AD7291 VINX.

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