staging:iio:meter: Replaces IIO_DEV_ATTR_CH_OFF by IIO_DEVICE_ATTR

The macro IIO_DEV_ATTR_CH_OFF is a wrapper for IIO_DEVICE_ATTR, with a
tiny change in the name definition. This extra macro does not improve
the readability and also creates some checkpatch errors.

This patch fixes the checkpatch.pl errors:

staging/iio/meter/ade7753.c:391: ERROR: Use 4 digit octal (0777) not
decimal permissions
staging/iio/meter/ade7753.c:395: ERROR: Use 4 digit octal (0777) not
decimal permissions
staging/iio/meter/ade7759.c:331: ERROR: Use 4 digit octal (0777) not
decimal permissions
staging/iio/meter/ade7759.c:335: ERROR: Use 4 digit octal (0777) not
decimal permissions

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>