mlxsw: spectrum: Distinguish between unsplittable and split port

Currently when user does split, he is not able to distinguish if the
port cannot be split because it is already split, or because it cannot
be split at all. Add another check for split flag to distinguish this.
Also add check forbidding split when maximal width is 1.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Shalom Toledo <shalomt@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>