media: common/saa7146: fix VFL direction for vbi output

The VBI output device didn't have VFL_DIR_TX set, so this didn't work
anymore since the V4L2 core thought that it was a capture device instead.

Fix this. Also drop invalid capabilities for the VBI output device.

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