net: dsa: mv88e6xxx: rework port state setter

Apply a few non-functional changes on the port state setter:

  * add a dynamic debug message with state names to track changes
  * explicit states checking instead of assuming their numeric values
  * lock mutex only once when changing several port states
  * use bitmap macros to declare and access port_state_update_mask

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>