net: bridge: Extract boilerplate around switchdev_port_obj_*()

A call to switchdev_port_obj_add() or switchdev_port_obj_del() involves
initializing a struct switchdev_obj_port_vlan, a piece of code that
repeats on each call site almost verbatim. While in the current codebase
there is just one duplicated add call, the follow-up patches add more of
both add and del calls.

Thus to remove the duplication, extract the repetition into named
functions and reuse.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>