[media] e4000: fix crash when attach fails

Callbacks were set even attach failed. This leads calling
.release() in error case and resulted crash.

Reported-by: Oliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>