net/mlx5: Honor user input for migratable port fn attr

Currently, whenever a user is setting migratable port fn attr, the
driver is always turn migratable capability on.
Fix it by honor the user input

Fixes: e5b9642a

 ("net/mlx5: E-Switch, Implement devlink port function cmds to control migratable")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>