iwlwifi: only show active power level via sysfs

This changes the power_level file to adhere to the "one value
per file" sysfs rule. The user will know which power level was
requested as it will be the number just written to this file. It
is thus not necessary to create a new sysfs file for this value.

In addition it fixes a problem where powertop's parsing expects
this value to be the first value in this file without any descriptions.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>