i2c: piix4: Fully initialize SB800 before it is registered

This closes a race window where I2C device drivers attempt to access
I2C buses which aren't fully initialized yet.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Tested-by: Christian Fetzer <fetzer.ch@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>