[media] go7007: remember boot firmware

Don't load it everytime you stop encoding. Instead remember it.
Another reason for not loading it every time is that this could
be called from within the release() file operation, which turns
out to be deadly.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>