s390/qeth: Remove pnso workaround

Remove workaround that supported early hardware implementations
of PNSO OC3. Rely on the 'enarf' feature bit instead.

Fixes: fa115adf

 ("s390/qeth: Detect PNSO OC3 capability")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
[jwi: use logical instead of bit-wise AND]
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>