Input: synaptics - fix setting packet size on passthrough port.

Synaptics driver used child->type to select either 3-byte or 4-byte
packet size for the pass-through port; this gives wrong results for
the newer protocols. Change the check to use child->pktsize instead.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>