net: dsa: move port state setters

Add a new port.c file to hold all DSA port-wide logic. This patch moves
in the code which sets a port state.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>