Input: appletouch - improve finger detection

The appletouch driver is prone to reporting multiple fingers when only
one is pressing.  The appletouch driver queries an array of pressure
sensors and counts local maxima in pressure to determine the number of
fingers.  It just does this on the raw values, so a data stream like:

0 100 250 300 299 300 250 100 0

actually registers as 2 fingers.

This patch updates the logic to ignore small dips in pressure that are
less than the threshold.

Signed-off-by: Jeremy Huddleston <jeremyhu@freedesktop.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>