Input: wacom - remove field pktlen declaration in the list of devices

pktlen is now overwritten by the driver directly by reading the hid
report descriptor. There is no need to declare it statically.
We also move down the position of the field in the struct so that
we can keep the current declaration of Wacom devices.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>