V4L/DVB (6900): cx23885: enable EZ-QAM mode for Hauppauge WinTV HVR-1800

Add module option 'alt_tuner' disabled by default.

When set to one, the dvb_frontend of HVR1800 will consist of:
s5h1409 demod + tda18271 tuner

When set zero (default), the dvb_frontend of HVR1800 will consist of:
s5h1409 demod + mt2131 tuner

If the tda18271 is used in digital mode, you will not be able to
tune an analog channel at the same time.

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