zebra: minor changes on "zebra_evpn_mac_gw_macip_add" function

zebra: minor changes on "zebra_evpn_mac_gw_macip_add" function

Two minor changes:
    1) Change `zebra_evpn_mac_gw_macip_add()` 's return type to `void`.
    2) Since `zebra_evpn_mac_gw_macip_add()` has already `assert` the returned
    `mac`, the check of its return value makes no sense. And keep setting
    `mac->flags` inside `zebra_evpn_mac_gw_macip_add()` is more reasonable. So
    just move the setting `mac->flags` inside `zebra_evpn_mac_gw_macip_add()`.

Signed-off-by: anlan_cs <vic.lan@pica8.com>