HID: Add Tablet Pick -> BTN_STYLUS2 mapping

Many tablet input drivers use BTN_STYLUS2 code to report a second
tablet stylus button. Yet, the generic HID input driver doesn't map it.
Apparently, because there is no corresponding usage in the HID Usage Tables.

This patch selects a compromise usage - "Tablet Pick" (0x46) to be mapped to
BTN_STYLUS2. Some tablets use "Eraser" usage, but mapping it to a button
wouldn't be entirely correct, because the specification says it may be "a
pressure quantity".

Signed-off-by: Nikolai Kondrashov <spbnick@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-input.c

@@ -336,6 +336,10 @@ static void hidinput_configure_usage(struct hid_input *hidinput, struct hid_fiel
 			map_key_clear(BTN_STYLUS);
 			break;
 
+		case 0x46: /* TabletPick */
+			map_key_clear(BTN_STYLUS2);
+			break;
+
 		default:  goto unknown;
 		}
 		break;