V4L/DVB (3673): Fix budget-av CAM reset

Unfortunately on the budget-av board, the CAM reset line is tied to the
frontend reset line, so resetting the CAM also zaps the frontend. This
breaks the tda1004x at least, and causes it to fail to tune until the
budget-av module is reloaded. This patch adds an exported function to dvb_frontend
that allows a card to forcibly reinitialise a frontend. The budget-av now
does this on CAM reset, which corrects this problem.
since they do not tie the CAM reset line to the frontend reset line.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>