net: dsa: mv88e6xxx: Move mv88e6xxx_port_db_load_purge()

This function is going to be needed by a soon to be added new
function. Move it earlier so we can avoid a forward declaration.
No functional changes.

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>