[media] em28xx: save isoc endpoint number for DVB only if endpoint has alt...

[media] em28xx: save isoc endpoint number for DVB only if endpoint has alt settings with xMaxPacketSize != 0

X-Patchwork-Delegate: mchehab@redhat.com
In addition to commit 72cc9ba3 "em28xx: ignore isoc DVB USB endpoints with
wMaxPacketSize = 0 bytes for all alt settings" we should not save the endpoint
number of the isoc DVB endpoint before it has been validated.
While the current code works fine, dev->dvb_ep_isoc != 0 could be interpreted
as indicator that the device provides DVB support.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>