V4L/DVB (7689): pvrusb2-dvb: Rework module tear-down

Rather than making an explicit call to tear down the pvrusb2-dvb
module, use the callback in the pvr2_channel structure.  This has the
advantage that now tear-down only happens when it makes sense.  The
previous implementation had scenarios where it was possible for the
tear-down call to happen without a prior initialization.

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