net/mlx4: Support shutdown() interface

In kexec scenario, we failed to load the mlx4 driver in the
second kernel because the ownership bit was hold by the first
kernel without release correctly.

The patch adds shutdown() interface so that the ownership can
be released correctly in the first kernel. It also helps avoiding
EEH error happened during boot stage of the second kernel because
of undesired traffic, which can't be handled by hardware during
that stage on Power platform.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Tested-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>