s390/qeth: also use TX NAPI for non-IQD devices

Set scan_threshold = 0 to opt out from the qdio layer's internal tasklet
& timer mechanism for TX completions, and replace it with the TX NAPI
infrastructure that qeth already uses for IQD devices. This avoids the
fragile logic in qdio_check_output_queue(), enables tighter integration
and gives us more tuning options via ethtool in the future.

For now we continue to apply the same policy as the qdio layer:
scan for completions if 32 TX buffers are in use, or after 1 sec.
A re-scan is done after 10 sec, but only if no TX interrupt is pending.

With scan_threshold = 0 we no longer get TX completion scans from
within qdio_get_next_buffers(). So trigger these manually in qeth_poll()
and in the RX path switch to the equivalent qdio_inspect_queue().

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>