i2c: immediately mark ourselves as registered

Mark the i2c bus as registered right after the the bus_register call,
not at the end of init. Otherwise, we can't register our own dummy
driver.

Reported-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Fixes: 95026658 ("i2c: do not use internal data from driver core")