staging: iio: replace symbolic permission

This patch fixes the following checkpatch.pl warning in ad7150.c
WARNING: Symbolic permissions 'S_IRUGO | S_IWUSR' are not preferred
Occured during build.

Signed-off-by: Amit Kushwaha <kushwaha.a@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>