xkb: fix testing if on keypad

XKB_KEY_KP_9 is 0xffb9 while XKB_KEY_KP_Equal is 0xffbd and XKB_KEY_F1
is 0xffbe. So XKB_KEY_KP_Equal, instead of XKB_KEY_KP_9, has the maximum
keysym for keypad keys.


(cherry picked from commit 681752ad)