net: dsa: lan9303: Add STP ALR entry on port 0

STP BPDUs arriving on user ports must sent to CPU port only,
for processing by the SW bridge.

Add an ALR entry with STP state override to fix that.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>