staging: iio: adc: ad7280a: fix permission coding style issue

Symbolic permissions 'S_IWUSR | S_IRUGO' are not preferred. Consider using
octal permissions '0644'. Found running checkpatch.

Signed-off-by: Olivier Leveque <o_leveque@orange.fr>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>