wifi: iwlwifi: Ensure ack flag is properly cleared.

Debugging indicates that nothing else is clearing the info->flags,
so some frames were flagged as ACKed when they should not be.
Explicitly clear the ack flag to ensure this does not happen.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Acked-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230808205605.4105670-1-greearb@candelatech.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>