HID: wacom: Initialize MT slots for generic devices at post_parse_hid

If a HID descriptor places HID_DG_CONTACTID before HID_DG_X and HID_DG_Y then
the ABS_X and ABS_Y will not be automatically initialized by the call to
input_mt_init_slots. To ensure that this is not a problem, we relocate that
call to occur after HID parsing has been completed and we've initalized all the
multitouch axes.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>