Input: bcm5974 - improve finger tracking and counting

The problem of finger tracking, i.e., when to switch focus from one
finger to another on the trackpad, has been improved by utilizing more
information from the bcm5974 chip output. This results in less pointer
hopping when many fingers are on the trackpad. In addition, a finger
counting method based on pressure information from all fingers is
introduced. Together with a pressure hysteresis window, this yields a
more stable counting of the number of fingers on the trackpad.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>