Staging: iio: gyro: Added description for config symbol.

The following patch fixes the checkpatch.pl warning by adding
description for a config symbol.
WARNING: please write a paragraph that describes the config symbol
fully.

Signed-off-by: Heena Sirwani <heenasirwani@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>