V4L/DVB (7070): Fix some tuning problems

The attached patch solves all my vdr tuning problems on a dib7000p
nova-t stick as far as I could check within the last weekend.

It disables streaming while tuning, like that the number of faulty TS
packets is reduced.

Signed-off-by: Soeren Moch <Soeren.Moch@stud.uni-hannover.de>
Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>