net: dsa: mv88e6xxx: lock mutex in port_fdb_dump

During a port FDB dump operation, the mutex protecting the concurrent
access to the switch registers is currently held by the internal
mv88e6xxx_port_db_dump and mv88e6xxx_port_db_dump_fid helpers.

It must be held at the higher level in mv88e6xxx_port_fdb_dump which
is called directly by DSA through ds->ops->port_fdb_dump. Fix this.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>