iwlwifi: error checking for setting tx_power in sysfs

Perform error checking and report failure when setting tx power from
sysfs.
If fail to set the tx power, do not update the local copy, so user will
not see the incorrect tx power in sysfs

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>