s390/qeth: clean up error handling for isolation mode cmds

As the cmd IO path has learned to propagate errnos back to its callers,
let them deal with errors instead of trying to restore their previous
configuration from within the IO error path.

Also translate the HW error to a meaningful errno, instead of returning
-EIO for all cases (and don't map this to -EOPNOTSUPP later on...).

While at it, add a READ_ONCE() / WRITE_ONCE() pair to ensure that the
data path always sees a valid isolation mode during reconfiguration.

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