V4L/DVB (7300): pvrusb2: v4l2 implementation fixes for input selection

Now that the pvrusb2 driver can dynamically choose which inputs to
make available depending on the hardware, the enumeration of input
choices is no longer a contiguous range of integers.  Unfortunately
this causes a problem in the v4l2 implementation since the input
enumeration requires continuity in the API.  This change implements a
mapping in order to preserve the v4l2 interface requirement.

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