[media] media: fix truncated entity specification

When enumerating an entity, assign the entire entity specification
instead of only the first two words. (This requires giving the
specification union a name.)

So far, no driver actually uses more than two words, but this will
be needed for ALSA entities.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
[laurent.pinchart@ideasonboard.com: Rename specification to info]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>