iwlwifi: corrects power_level in sysfs

This patch corrects power_level in sysfs.

Signed-off-by: Esti Kummer <ester.kummer@intel.com>
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>