[media] pvrusb2: Handle change of mode before handling change of video standard

pvrusb2: Recognize and handle mode change before dealing with changes
related to the video standard.  Even though the video standard should
only matter when in analog mode, doing this way is technically cleaner
in case there's other stuff that might depend on both.

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