Input: ads7846 - report 0 pressure value along with pen up event

X touchscreen drivers that don't interpret the designated pen up message
assume a pen up event from a pressure value 0. For these we generate a
pressure 0 message along with the pen up message.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Acked-by: Juha Yrjola <juha.yrjola@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>