Don't hold state lock while detaching network device instance in mlx4en(4).

It can happen during shutdown that the lock will recurse when the mlx4en(4)
instance is part of a lagg interface. Call ether_ifdetach() unlocked.

Backtrace:
panic(): _sx_xlock_hard: recursed on non-recursive sx &mdev->state_lock
_sx_xlock_hard()
_sx_xlock()
mlx4_en_ioctl()
if_setlladdr()
lagg_port_destroy()
lagg_port_ifdetach()
if_detach()
mlx4_en_destroy_netdev()
mlx4_en_remove()
mlx4_remove_device()
mlx4_unregister_device()
mlx4_unload_one()
mlx4_shutdown()
linux_pci_shutdown()
bus_generic_shutdown()

MFC after: 1 week
Sponsored by: Mellanox Technologies