s390/qeth: clean up CREATE_ADDR cmd code

Properly define the cmd's struct to get rid of some casts and accesses
at magic offsets.

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