net/mlx4: remove limitation on number of instances

net/mlx4: remove limitation on number of instances

The seemingly artificial limitation on the maximum number of instances for
this PMD is an historical leftover that predates its first public release.

It was used as a workaround to support multiple physical ports on a PCI
device exposing a single bus address when mlx4 was implemented directly as
an Ethernet device driver instead of a PCI driver spawning Ethernet
devices.

Getting rid of it simplifies device initialization.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>