net: ocelot: export ocelot_ifh_port_set() to setup IFH

FDMA will need this code to prepare the injection frame header when
sending SKBs. Move this code into ocelot_ifh_port_set() and add
conditional IFH setting for vlan and rew op if they are not set.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>