qeth: replace ENOSYS with EOPNOTSUPP

Since recently, `checkpatch.pl` advices that ENOSYS should not be
used for anything other than "invalid syscall nr". This patch
replaces ENOSYS return code with EOPNOTSUPP for the "unsupported
function" conditions.

Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>