i2c-i801: Handle multiple instances instead of keeping global state

It's poor form to keep driver state in global variables rather than
per-instance. It never really mattered in practice when there was only
one controller on the chipset, but the latest chipsets do have more
than one controller, so now we care.
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>