bonding: fix socket id for LACP slave

On slave activation in LACP (8023AD) SOCKET_ANY_ID (which is -1)
is being casted to unsigned char and then to signed int.
The result is that socket_id has value of 255, not -1.
This results to memory allocation failure.

Fixes: 46fb43683679 ("bond: add mode 4")

Signed-off-by: Sergey Balabanov <balabanovsv@ecotelecom.ru>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>