Bluetooth: Rename sysfs un/register to add/del

As we introduced hci_init_sysfs() we should also rename
hci_register_sysfs() and hci_unregister_sysfs() to hci_add_sysfs() and
hci_del_sysfs() like we do with hci_conn_add/del_sysfs(). It looks more
consistent now.

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