lldp: fix memory leakage

lldp: fix memory leakage

1. Typo in usage of vnet_hw_interface_add_del_mac_address(),
   which returns 0 when it succeeds instead non zero value.

2. Generated error doesn't clean allocated resources for
   an interface.

3. Returned value from vnet_hw_interface_add_del_mac_address()
   should be erased or reported.

Type: fix
Fixes: 149fd3fbd069a5f7be86e68472578ee7af229cb6

Signed-off-by: Dmitry Vakhrushev <dmitry@netgate.com>
Change-Id: Ia6b28ae70fea127d15eb0102223ff972358766bc
Signed-off-by: Dmitry Vakhrushev <dmitry@netgate.com>