iwlwifi: use strict_strtoul instead of simple_strtoul

This patch makes use of strict_srtoul in sysfs code.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>