staging: iio: adc: ad7280a: Add comments to clarify stringified arguments

Checkpatch would flash a check message around a stringified macro
argument containing a '-' character. Add comment to indicate the
argument is legitimate and doesn't need fixing.

Signed-off-by: Deepak R Varma <mh12gx2825@gmail.com>
Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/469bf8db1c228913e72841367182fba2168fe795.1584904896.git.mh12gx2825@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>