Input: alps - only set fields that are actually present

Pinnacle / Rushmore packets contain either position info, or bitmap info,
never both. So far we've in essence been storing garbage in the position /
bitmap fields of the fields struct when decoding a bitmap / pos packet.

We've been relying on the following sequence to get away with this:

1) Decode bitmap packet
2) Process bitmap packet
3) Decode position packet
4) Use position / button info

This patch allows us to change this sequence, which will allow using the
position info when processing the bitmap for more accurate results.

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