Bluetooth: Forward errors from hci_register_dev

We need to catch errors when calling hci_add_sysfs() and return them to
the caller to avoid kernel oopses on device_add() failure.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>