i2c: Fix bad hint about irqs in i2c.h

i2c.h mentions -1 as a not-issued irq. This false hint was taken by
of_i2c and caused crashes. Don't give any advice as 'no irq' is not
consistent across all architectures yet and it is not needed internally
by the i2c-core.
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

include/linux/i2c.h

@@ -171,7 +171,7 @@ struct i2c_client {
 	struct i2c_adapter *adapter;	/* the adapter we sit on	*/
 	struct i2c_driver *driver;	/* and our access routines	*/
 	struct device dev;		/* the device structure		*/
-	int irq;			/* irq issued by device (or -1) */
+	int irq;			/* irq issued by device		*/
 	struct list_head list;		/* DEPRECATED */
 	struct completion released;
 };