V4L/DVB (6698): pvrusb2: Implement signal routing schemes

The exact routing of video and audio signals within a device is a
device-specific attribute.  Hauppauge devices do it one way; other
types of device may route things differently.  Unfortunately it is
rather impractical to define chip-specific routing at the device
attribute level, so instead what happens here is that "schemes" are
defined.  Each chip level interface implements its part of a given
scheme and the scheme as a whole is made into a device specific
attribute controlled via a table entry in pvrusb2-devattr.c.  The only
scheme defined here is for Hauppauge devices, but clearly this opens
the door for other possibilities to follow.

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