net/mlx5_core: Apply proper name convention to helpers

Some core helper functions were named with mlx5_ only prefix, fix that to
mlx5_core_ so we're aligned with the overall scheme used for core services.

Signed-off-by: Haggai Abramovsky <hagaya@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>