net/mlx5: Move esw multiport devlink param to eswitch code

Move the param registration and handling code into the eswitch
code as they are related to each other. No point in having the
devlink param registration done in separate file.

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