HID: multitouch: use proper symbolic constant for 0xff310076 application

0xff310076 application has been defined by 957b8dff ("HID: multitouch:
Support Asus T304UA media keys") as a vendor-specific application with
symbolic constant HID_VD_ASUS_CUSTOM_MEDIA_KEYS, so let's make use of
it.

Fixes: 957b8dff ("HID: multitouch: Support Asus T304UA media keys")
Signed-off-by: Jiri Kosina <jkosina@suse.cz>