net: dsa: refactor the code to set the port MAC address into a dedicated function

net: dsa: refactor the code to set the port MAC address into a dedicated function

This snippet of code has a bothering "if (...) return 0" in it which
assumes it is the last piece of code running in dsa_port_probe().

This makes it difficult to add further code at the end of dsa_port_probe()
which does not depend on MAC address stuff.

So move the code to a dedicated function which returns void and let the
code flow through.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Tested-by: Michael Walle <michael@walle.cc>