V4L/DVB (12517): mt9t031: improve rectangle placement in invalid S_CROP

1. soc-camera always requests a valid rectangle, when calling .s_fmt(), no need
to check and adjust
2. in .s_crop(), if the rectangle exceeds sensor limits, push it to the
respective border instead of centering
3. take into account left and top borders when checking

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>