i2c: Allow i2c_add_numbered_adapter() to assign a bus id

Currently, if an i2c bus driver supports both static and dynamic bus
ids, it needs to choose between calling i2c_add_numbered_adapter() and
i2c_add_adapter().  This patch makes i2c_add_numbered_adapter()
redirect to i2c_add_adapter() if the requested bus id is -1.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Jean Delvare <khali@linux-fr.org>