Input: ad7879 - prevent invalid finger data reports

Considering following scenario - the touch is present on the screen
at the beginning of the last conversion sequence, but by the time
the last sequence is finished, the finger is lift off. The AD7879 data
available interrupt signals (DAV) completion, however some X,Y values
are not valid because the screen inputs were floating during the
acquisition.

The AD7877 acts differently here, since it only asserts DAV if the
touch is still present when the conversion sequence finished.

Based on the fact that this can only happen in the last sample of the
repeated conversion sequence, we simply skip the last (short glitches
are filtered by the AD7879 internal median and average filters).
This doesn't cause noticeable side effects, since the minimum conversion
interval is 9.44ms. We receive ~100 waypoint samples per second, so we
simply delay the result by 9.44ms.

We also reject samples where pressure is greater than pressure_max.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>