s390/qeth: let isolation mode override HW offload restrictions

When a device is configured with ISOLATION_MODE_FWD, traffic never goes
through the internal switch. Don't apply the offload restrictions in
this case.

Fixes: c619e9a6 ("s390/qeth: don't use restricted offloads for local traffic")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>