[media] em28xx: Reworked probe code to get rid of some hacks

Reworked device probing to get rid of hacks to guess the maximum size of
dvb iso transfer packets. The new code also selects the first alternate
config which supports the largest possible iso transfers for dvb.

[mchehab@redhat.com: Fix a few checkpatch.pl CodingStyle compliants]
Signed-off-by: Holger Nelson <hnelson@hnelson.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>