V4L/DVB (6709): pvrusb2: minor rework for default video standard handling

pvrusb2: When a per-device-type default video standard is declared,
handle it in such a way that it can be correctly and unambiguously
reported in the system log.

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