RDMA/mlx5: Limit the scope of mlx5_ib_enable_driver function

The mlx5_ib_enable_driver() is local function and doesn't need to be
shared in mlx5_ib, so change it's signature to have static keyword in it.

Link: https://lore.kernel.org/r/20200702081809.423482-2-leon@kernel.org


Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>