qeth: return with -EPERM if sniffing is not enabled

Without appropriate configuration at the SE, a HiperSockets device
cannot be used for sniffing. Setting the sniffer attribute is rejected
with -EPERM.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>