iwlwifi: introduce statistics lock

The statistics are currently only half-heartedly
locked against concurrent reading & modification
so introduce a lock to really protect them.

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