Input: alps - fix reporting pressure of v3 trackstick

According to documentation, all 7 lower bits represents trackpoint pressure.

Fixes: 4621c966 ("Input: alps - report pressure of v3 and v7 trackstick")
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>