iwlwifi: mvm: take RCU lock before dereferencing

RCU isn't properly locked.

Fixes: 46d372af ("iwlwifi: mvm: rs: new rate scale API - add FW notifications")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>