net: dsa: mv88e6xxx: Move forward declaration to where it is needed

Move it out from the middle for the #defines to just before it is
needed.

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