V4L/DVB (4643): Multi-input patch for DVB-USB device

This patch is the first commit of the Multiple Input Patch for the DVB-USB frame
work.
It changes the DVB-USB-device to be able to have more than one streaming input
(e.g. multiple DVB-T sources) on one device. This is a necessary feature for
the upcoming DiB7700 driven devices.

Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>