RDMA/mlx5: Limit scope of get vector affinity local function

The mlx5_ib_get_vector_affinity() call is local to main.c file and there
is no need to be declared globally visible.

Fixes: 40b24403 ("mlx5: support ->get_vector_affinity")
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>