Input: atmel_tsadcc - improve accuracy

Discard the last sample just before pen is up because it is quite often
errorneous.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dan Liang <dan.liang@atmel.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>