V4L/DVB (5423): M920x: i2c cleanups

- Implement m920x i2c as suggested by Pierre Willenbrock
- remove "magic" hack
- r/w bit is not part of the i2c address
- move hardware remarks to header file

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