mlx4: Implement port type setting via devlink interface

So far, there has been an mlx4-specific sysfs file allowing user to
change port type to either Ethernet of InfiniBand. This is very
inconvenient.

Allow to expose the same ability to set port type in a generic way
using devlink interface.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>