s390/qeth: replace deprecated simple_stroul()

Convert the remaining occurences in sysfs code to kstrtouint().

While at it move some input parsing out of locked sections, replace an
open-coded clamp() and remove some unnecessary run-time checks for
ipatoe->mask_bits that are already enforced when creating the object.

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