net: dsa: deduplicate code adding / deleting the port address to fdb

The sequence
  if (dsa_switch_supports_uc_filtering(ds))
    dsa_port_standalone_host_fdb_add(dp, addr, 0);
  if (!ether_addr_equal(addr, conduit->dev_addr))
    dev_uc_add(conduit, addr);
is executed both in dsa_user_open() and dsa_user_set_mac_addr().

Its reverse is executed both in dsa_user_close() and
dsa_user_set_mac_addr().

Refactor these sequences into new functions dsa_user_host_uc_install()
and dsa_user_host_uc_uninstall().

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>