[media] DocBook: bus_info can no longer be empty

During the 2012 Media Workshop it was decided that bus_info as returned
by VIDIOC_QUERYCAP can no longer be empty. It should be a unique identifier,
and empty strings are obviously not unique.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>