devlink: Enhance policy to validate eswitch mode value

Use range checking facility of nla_policy to validate eswitch mode input
attribute value is valid or not.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>