net: caif: avoid using qdisc_qlen()

Such helper does not cope correctly with NOLOCK qdiscs.
In the following patches we will move back qlen to per CPU
values for such qdiscs, so qdisc_qlen_sum() is not an option,
too.
Instead, use qlen only for lock qdiscs, and always set
flow off for NOLOCK qdiscs with a not empty tx queue.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>