mlxsw: destroy workqueue when trap_register in mlxsw_emad_init

When mlxsw_core_trap_register fails in mlxsw_emad_init,
destroy_workqueue() shouled be called to destroy mlxsw_core->emad_wq.

Fixes: d965465b

 ("mlxsw: core: Fix possible deadlock")
Signed-off-by: Liu Jian <liujian56@huawei.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>