net: dsa: resolve tagging protocol at parse time

Extend the dsa_port_parse_cpu() function to resolve the tagging protocol
at port parsing time, instead of waiting for the whole tree to be
complete.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>