s390/qeth: consolidate qeth MAC address helpers

For adding/removing a MAC address, use just one helper each that
handles both unicast and multicast.
Saves one level of indirection for multicast addresses, while improving
the error reporting for unicast addresses.

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