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: ea7f3cfa

 ("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>