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: Wolfram Sang <[e-mail]>
Signed-off-by: Jean Delvare <[e-mail]>

include/linux/i2c.h

···
 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;