[media] DocBook: media: struct v4l2_capability card field is a UTF-8 string

The struct v4l2_capability card field stores the device name. That name
can be hardcoded in drivers, or be retrieved directly from the device.
The later is very common with USB devices. As several devices already
report names that include non-ASCII characters, update the field
description to use UTF-8.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>