s390/qeth: don't return -ENOTSUPP to userspace

ENOTSUPP is not uapi, use EOPNOTSUPP instead.

Fixes: d66cb37e ("qeth: Add new priority queueing options")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>