net: dsa: change scope of STP state setter

Instead of having multiple STP state helpers scoping a slave device
supporting both the DSA logic and the switchdev binding, provide a
single dsa_port_set_state helper scoping a DSA port, as well as its
dsa_port_set_state_now wrapper which skips the prepare phase.

This allows us to better separate the DSA logic from the slave device
handling.

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>