[media] saa7134-dvb: Fix kworld sbtvd I2C gate control

The correct place to put i2c_gate_ctrl is before calling tda18271_attach,
because the driver tda18271 will use it to enable or disable the i2c-bus
from the demodulator to the tuner.

And thus eliminate the error message: "Unknown device (255) detected
@ 1-00c0, device not supported" in the driver tda18271.

In the device kworld_sbtvd (hybrid analog and digital TV) the control
of the i2c-bus to tuner is done in the analog demodulator and not in
the digital demodulator.

Signed-off-by: Manoel Pinheiro <pinusdtv@hotmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>