V4L/DVB (9178): cx24116: Add module parameter to return SNR as ESNO.

I've retained the older percentage code after discussing this with
Darron Broad, but made the ESNO option default.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>