iwlwifi: fix thermal throttling related power management operation

The current approach is very broken because it adds an
often-used code path that will not initialise "cmd" at all.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Shanyu Zhao <shanyu.zhao.intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>