net/mlx5: E-switch, Remove dependency between sriov and eswitch mode

Currently, there are three eswitch modes, none, legacy and switchdev.
None is the default mode. Remove redundant none mode as eswitch mode
should always be either legacy mode or switchdev mode.

With this patch, there are two behavior changes:

1. Legacy becomes the default mode. When querying eswitch mode using
   devlink, a valid mode is always returned.
2. When disabling sriov, the eswitch mode will not change, only vfs
   are unloaded.

Signed-off-by: Chris Mi <cmi@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>