V4L/DVB (5779): Dibusb-mb: fix broken 'tuner_pass_ctrl' functionality

'tuner_pass_ctrl' functionality of the dib3000-mb devices was broken in 
the previous changeset:

"dibusb-mb: convert pll handling to properly use dvb-pll"

This patch fixes this problem by assigning this functionality to the 
i2c_gate_ctrl callback

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