s390/qeth: clean up qeth_set_ecmd_adv_sup()

In preparation for moving to get_link_ksettings(), clean up how
we build the supported and advertised port/speed masks.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Reviewed-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>