V4L/DVB (7615): em28xx: Provide the proper support for switching between analog/digital

Before this patch,  HVR900/HVR950 were incorreclty going back to analog. The
result is that only digital were working.

This patch provides the proper setup for analog/digital and tuner callback.
It also properly resets analog into a sane state at open().

Thanks to Steven Toth <stoth@linuxtv.org> and Michael Krufky <mkrufky@linuxtv.org>
for helping to set the proper parameters to GPO/GPIO em2883 ports.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>