mlxsw: spectrum_switchdev: Don't batch STP operations

Simplify the code by using the common function that sets an STP state
for a Port-VLAN and remove the existing one that tries to batch it for
several VLANs.

This will help us in a follow-up patchset to introduce a unified
infrastructure for bridge ports, regardless if the bridge is VLAN-aware
or not.

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