V4L/DVB (6200): cx23885: use a pointer to the required i2c bus in dvb_register function

Store a pointer to the required i2c_bus so that we do not put the wrong
analog demod into standby.

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