V4L/DVB (5174): Pvrusb2: video corruption fixes

Tweak the encoder setup in order to stop it from corrupting the video
data when there is a disruption in the data flow (e.g. a channel change).

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>