devlink: don't take instance lock around eswitch mode set

Changing switch mode may want to register and unregister devlink
ports.  Therefore similarly to DEVLINK_CMD_PORT_SPLIT/UNSPLIT it
should not take the instance lock.  Drivers don't depend on existing
locking since it's a very recent addition.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>