i2c: Make clear what the class field of i2c_adapter is good for

Make clear what the class field of i2c_adapter is good for.
Signed-off-by: NWolfram Sang <w.sang@pengutronix.de>
Signed-off-by: NJean Delvare <khali@linux-fr.org>

include/linux/i2c.h

@@ -345,7 +345,7 @@ struct i2c_algorithm {
 struct i2c_adapter {
 	struct module *owner;
 	unsigned int id;
-	unsigned int class;
+	unsigned int class;		  /* classes to allow probing for */
 	const struct i2c_algorithm *algo; /* the algorithm to access the bus */
 	void *algo_data;