V4L/DVB (4946): Cx88: Move cx88_dvb_bus_ctrl out of the card-specific area

This function should not be in the card-specific area of the file.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>