Input: mpr121 - set missing event capability

This driver reports misc scan input events on the sensor's status
register changes.  But the event capability for them was not set in the
device initialization, so these events were ignored.

This change adds the missing event capability.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>