s390/qeth: fix initialization on old HW

I stumbled over an old OSA model that claims to support DIAG_ASSIST,
but then rejects the cmd to query its DIAG capabilities.

In the old code this was ok, as the returned raw error code was > 0.
Now that we translate the raw codes to errnos, the "rc < 0" causes us
to fail the initialization of the device.

The fix is trivial: don't bail out when the DIAG query fails. Such an
error is not critical, we can still use the device (with a slightly
reduced set of features).

Fixes: 742d4d40

 ("s390/qeth: convert remaining legacy cmd callbacks")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>