net/mlx5: Separate between public and private API of sf.h

Move mlx5_sf_max_functions() and friends from the privete sf/sf.h
to the public lib/sf.h. This is done in order to have one direction
include paths.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>