[media] videodev2.h: add parenthesis around macro arguments

bt->width should be (bt)->width, and same for the other fields.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: stable@vger.kernel.org # For 3.12 or upper
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>