qeth: set promisc off after trace disabling failure

If HiperSockets Network Traffic Analyzer is switched off, but trace
disabling fails somehow, the qeth driver does not switch off its
promisc mode status. A following sniffer reactivation fails, since
qeth does not see a need to reenable tracing.
At the same time the code analyzing results of trace commands is
restructured.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>