V4L/DVB (6449): tda18271: clean up i2c_gate handling

Call analog_demod_ops->i2c_gate_ctrl when in analog tuning mode, and
frontend_ops.i2c_gate_ctrl when in digital tuning mode.

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