net/mlx5: Remove devl_unlock from mlx5_devlink_eswitch_mode_set

The callback mlx5_devlink_eswitch_mode_set() had unlocked devlink as a
temporary workaround once devlink instance lock was added to devlink
eswitch callbacks. Now that all flows triggered by this function
that took devlink lock are using devl_ API and all parallel paths are
locked we can remove this workaround.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>