A number of different ALPS touchpad protocols can reuse
alps_process_touchpad_packet_v3() with small tweaks to the bitfield
decoding.  Create a new priv->decode_fields() callback that handles the
per-model differences.
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Tested-by: Dave Turvene <dturvene@dahetral.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>