Input: wacom - separate pen from express keys on Graphire

Since Graphire/Bamboo devices report pen and expresskeys in the same
data packet, we need to send a input_sync event to separate pen data
from expresskeys for X11 driver to process them properly.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>