bridge: netlink: account for the IFLA_BRPORT_PROXYARP_WIFI attribute size and policy

The attribute size wasn't accounted for in the get_slave_size() callback
(br_port_get_slave_size) when it was introduced, so fix it now. Also add
a policy entry for it in br_port_policy.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Fixes: 842a9ae0 ("bridge: Extend Proxy ARP design to allow optional rules for Wi-Fi")
Signed-off-by: David S. Miller <davem@davemloft.net>