[media] em28xx: Don't initialize a var if won't be using it

[media] em28xx: Don't initialize a var if won't be using it

Fixes most cases of initializing a var but not using it.

There are still 3 cases at em28xx-alsa, were those vars should
probably be used.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>