staging: comedi: addi_apci_2032: document the spinlock_t definition

Add some comments to the private data tp quiet the checkpatch.pl
issue about:

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/addi_apci_2032.c

@@ -44,9 +44,9 @@
 #define APCI2032_WDOG_REG		0x10
 
 struct apci2032_int_private {
-	spinlock_t spinlock;
-	bool active;
-	unsigned char enabled_isns;
+	spinlock_t spinlock;		/* protects the following members */
+	bool active;			/* an async command is running */
+	unsigned char enabled_isns;	/* mask of enabled interrupt channels */
 };
 
 static int apci2032_do_insn_bits(struct comedi_device *dev,