s390/qeth: remove open-coded inet_make_mask()

Use inet_make_mask() to replace some complicated bit-fiddling.

Also use the right data types to replace some raw memcpy calls with
proper assignments.

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