s390/qeth: enable/disable the HW trap a little earlier

When setting a L2 qeth device online, enable the HW trap as soon as the
control plane is available. This allows us to catch any error that
occurs during the very first commands.

In the same spirit, the offline code should disable the HW trap as the
very first step of its processing.

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