sfc: Stop the TX queues during loopback self-tests

If the TX queues are running during loopback self tests, host
traffic gets looped back which causes the test to fail.  Avoid
restarting the TX queues after the port reset so that any packets
sent by the host get held back until after the tests have completed.

[bwh: Also wake all TX queues at the end of self-tests.]

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>