net: dsa: Define max num of bridges in tag8021q implementation

Max number of bridges in tag8021q implementation is strictly limited
by VBID size: 3 bits. But zero is reserved and only 7 values can be used.

This patch adds define which describe maximum possible value.

Suggested-by: Vladimir Oltean <[e-mail]>
Signed-off-by: Pawel Dembicki <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Vladimir Oltean <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/dsa/sja1105/sja1105_main.c

···
 	ds->vlan_filtering_is_global = true;
 	ds->untag_bridge_pvid = true;
 	ds->fdb_isolation = true;
-	/* tag_8021q has 3 bits for the VBID, and the value 0 is reserved */
-	ds->max_num_bridges = 7;
+	ds->max_num_bridges = DSA_TAG_8021Q_MAX_NUM_BRIDGES;
 
 	/* Advertise the 8 egress queues */
 	ds->num_tx_queues = SJA1105_NUM_TC;

include/linux/dsa/8021q.h

···
 #include <net/dsa.h>
 #include <linux/types.h>
 
+/* VBID is limited to three bits only and zero is reserved.
+ * Only 7 bridges can be enumerated.
+ */
+#define DSA_TAG_8021Q_MAX_NUM_BRIDGES	7
+
 int dsa_tag_8021q_register(struct dsa_switch *ds, __be16 proto);
 
 void dsa_tag_8021q_unregister(struct dsa_switch *ds);