V4L/DVB (6705): pvrusb2: Implement default standard selection based on device type

This adds a default video standard setting to the pvr2_device_desc
structure for describing device types.  With this change it is
possible to set a reasonable default standard based on device type.

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