Input: mma8450 - fix signed 12bits to 32bits conversion

Event value is wrong. Should be in range -2048 to 2047, but is in
range 0 to 4095.  Use s8 to int conversion and remove 0xfff mask.

Signed-off-by: Sebastien Royen <sebastien.royen@armadeus.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>