staging: comedi: amplc_pci224: fix spinlock_t definition without comment

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

drivers/staging/comedi/drivers/amplc_pci224.c

@@ -365,7 +365,7 @@ struct pci224_private {
 	const unsigned short *hwrange;
 	unsigned long iobase1;
 	unsigned long state;
-	spinlock_t ao_spinlock;
+	spinlock_t ao_spinlock;	/* spinlock for AO command handling */
 	unsigned int *ao_readback;
 	unsigned short *ao_scan_vals;
 	unsigned char *ao_scan_order;