V4L/DVB (5427): M920x: Improve I2C operations

Write some better documentation about what might be known about how the
m920x I2C works, since a datasheet is lacking.
The I2C xfer function should now handle more types of I2C transactions
than it could before.  Those it can't, will return error codes instead of
being executed incorrectly.  Multi-byte reads were not being done
correctly, which should be fixed.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>