Input: alps - change alps_decode_rushmore to do all decoding itself

Change alps_decode_rushmore to do all decoding itself, rather then relying
on alps_decode_pinnacle and then overriding some fields + or-ing in some
bits.

This is a preparation patch for modifying the decode functions to properly
differentiate between position and bitmap packets.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>