ads7846: filtering based on pressure value

Limit the pressure value to the maximum specified in the board-
specific file. Samples with pressure values beyond the maximum are
considered non-reliable. Report the previously measured X and Y
values samples instead of the non-reliable ones.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Juha Yrjola <juha.yrjola@nokia.com>