staging: comedi: amplc_dio200_common: document spinlock definition

Fix the checkpatch.pl issue:
CHECK: spinlock_t definition without comment
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/amplc_dio200_common.c

@@ -101,7 +101,7 @@ struct dio200_subdev_8255 {
 };
 
 struct dio200_subdev_intr {
-	spinlock_t spinlock;
+	spinlock_t spinlock;	/* protects the 'active' flag */
 	unsigned int ofs;
 	unsigned int valid_isns;
 	unsigned int enabled_isns;