V4L/DVB (4679): Fix for NULL pointer dereference oops during boot.

A fix for intermittent oops's during boot which occurs
in cx88_call_i2c_clients when dvb_attach is bringing up
the frontend.

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