iwlwifi: mvm: rs: don't zero tx stats after idle

Move the tx stats to the persistent area of lq_sta to
avoid them being zeroed out every time rs reinitializes
which happens after tx idle for 5 secs for example.
The automatic zeroing out made them difficult to use.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>