V4L/DVB (4291): Add dvbpll i2c device check.

Some cards have multiple possible addresses for their PLLs, with no other
way to tell if a PLL is present or not apart from probing to see if an i2c
device is present. This adds a quick check to see if an i2c device is
present at the given i2c address.

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