net/mlx4_core: Refactor mlx4_load_one

Refactor mlx4_load_one, as a preparation step for a new and
more complicated load function. The goal is to support both
newer firmware that required init_hca to be done before
enable_sriov and legacy firmwares that requires things to
be done the other way around.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>