Input: ALPS - remove assumptions about packet size

In preparation for version 4 protocol support, which has 8-byte
data packets, remove all hard-coded assumptions about packet size
and use psmouse->pktsize instead.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>