V4L/DVB (6801): tda18271: pass i2c gate configuration into tda18271_attach()

If we pass TDA18271_GATE_DIGITAL into tda18271_attach(), it will always try to
use the digital demodulator's i2c gate.

If we pass TDA18271_GATE_ANALOG into tda18271_attach(), it will always try to
use the analog demodulator's i2c gate.

If we pass TDA18271_GATE_AUTO into tda18271_attach(), it will try to use the
analog demodulator's i2c gate when tuning in analog mode, and it will try to
use the digital demodulator's i2c gate when tuning in digital mode.

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