media: common: saa7146: add support for missing .vidioc_try_fmt_vbi_cap

Support for VIDIOC_TRY_FMT for the vbi device was missing.
Add it.

This fixes a V4L2 compliance issue.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>