staging: comedi: mite: document the mite_struct spinlock_t

Add a comment to 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/mite.h

@@ -57,6 +57,7 @@ struct mite_struct {
 	short channel_allocated[MAX_MITE_DMA_CHANNELS];
 	int num_channels;
 	unsigned int fifo_size;
+	/* protects mite_channel from being released by the driver */
 	spinlock_t lock;
 };