net/mlx4_core: Check device state before unregistering it

Verify that the device state is registered before un-registering it.
This check is required to prevent an OOPS on flows that do
re-registration of the device and its previous state was
unregistered.

Fixes: 225c7b1f

 ("IB/mlx4: Add a driver Mellanox ConnectX InfiniBand adapters")
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>