[media] saa7146: fix compile warning

Fall-out from the recent struct v4l2_framebuffer change.

drivers/media/common/saa7146/saa7146_fops.c: In function ‘saa7146_vv_init’:
drivers/media/common/saa7146/saa7146_fops.c:536:6: warning: assignment from incompatible pointer type [enabled by default]
  fmt = &vv->ov_fb.fmt;
        ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>