ALPS v3 and v7 packet formats reports trackstick pressure. This information
is already parsed in unused "z" variable.

ALPS SS4 S2 devices already reports trackstick pressure as ABS_PRESSURE
attribute, therefore reports pressure in the same way also for v3 and v7.

This patch also updates parsing v3 pressure information, it is also stored
in 7 bits.
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>