iwlwifi: initialize rx_status

The vendor radiotap patch added a few fields to
struct ieee80211_rx_status that need to be zero,
initialize the struct instead of using whatever
was left on the stack.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>