V4L/DVB: tm6000: Properly set alternate when preparing to stream

Although the code is getting the better alternates, it is not really using
it. Get the interface/alternate numbers and use it where needed.

This patch implements also one small fix at the last_line set, as
proposed by Bee Hock Goh <behock@gmail.com>.

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