iio: adc: ad7192: fixed symbolic permission references 'S_IRUGO | S_IWUSR'

Replaced the symbolic permission references S_IRUGO and S_IWUSR with their
octal counterparts.

Signed-off-by: Artur Lorincz <larturus@yahoo.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>