Input: wacom - don't retrieve touch_max when it is predefined

Some models, such as 0xE6, report more fingers than we process.
Reported-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Tested-by: Nils Kanning <nils@kanning.de>
Tested-by: Rafi Rubin <rafi@seas.upenn.edu>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/tablet/wacom_sys.c

@@ -398,6 +398,8 @@ static int wacom_parse_hid(struct usb_interface *intf,
 				break;
 
 			case HID_USAGE_CONTACTMAX:
+				/* leave touch_max as is if predefined */
+				if (!features->touch_max)
 					wacom_retrieve_report_data(intf, features);
 				i++;
 				break;