Input: alps - process_bitmap: round down when spreading adjescent fingers over 2 points

This fixes 2 fingers at the same height or width on the touchpad getting
reported at different y / x coordinates.

Note num_bits is always at least 1.

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