i2c: Fix core-managed per-client debugfs handling

The debugfs directory should be created when a device
is probed, not when it is registered. It should be removed
when the device is removed, not when it is unregistered.

Fixes: d06905d6 ("i2c: add core-managed per-client directory in debugfs")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>