net: mscc: ocelot: refactor ocelot_port_inject_frame out of ocelot_port_xmit

The felix DSA driver will inject some frames through register MMIO, same
as ocelot switchdev currently does. So we need to be able to reuse the
common code.

Also create some shim definitions, since the DSA tagger can be compiled
without support for the switch driver.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>