dpaa2-switch: fix the translation between the bridge and dpsw STP states

The numerical values used for STP states are different between the
bridge and the MC ABI therefore, the direct usage of the
BR_STATE_* macros directly in the structures passed to the firmware is
incorrect.

Create a separate function that translates between the bridge STP states
and the enum that holds the STP state as seen by the Management Complex.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>