net: bql: fix building with BQL disabled

It is now possible to disable BQL, but that causes the cpsw driver to break:

drivers/net/ethernet/ti/am65-cpsw-nuss.c:297:28: error: no member named 'dql' in 'struct netdev_queue'
  297 |                    dql_avail(&netif_txq->dql),

There is already a helper function in net/sch_generic.h that could
be used to help here. Move its implementation into the common
linux/netdevice.h along with the other bql interfaces and change
both users over to the new interface.

Fixes: ea7f3cfaa5 ("net: bql: allow the config to be disabled")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -238,12 +238,7 @@ static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
 
 static inline int qdisc_avail_bulklimit(const struct netdev_queue *txq)
 {
-#ifdef CONFIG_BQL
-	/* Non-BQL migrated drivers will return 0, too. */
-	return dql_avail(&txq->dql);
-#else
-	return 0;
-#endif
+	return netdev_queue_dql_avail(txq);
 }
 
 struct Qdisc_class_ops {