V4L/DVB (4311): Fix possible dvb-pll oops

Supplying a NULL i2c adapter to dvb_pll_attach is allowed, for example with
mt352 demods. However, the pll i2c probe will segfault because it does not
check for this.

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