V4L/DVB: mx2_camera: implement forced termination of active buffer for mx25

This allows userspace to terminate a capture without waiting for the current
frame to complete.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>