iwlwifi: mvm: Free fw_status after use to avoid memory leak

fw_status is the only pointer pointing to a block of memory
allocated above and should be freed after use.
Note: this come from Klockwork static analyzer.

Cc: stable@vger.kernel.org [3.19+]
Fixes: 2021a89d ("iwlwifi: mvm: treat netdetect wake up separately")
Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>