iwlwifi: add sleep_tx_count ucode station API

This field was marked as reserved before since we didn't
use it, but is present in all released firmwares afaict.
We're going to need it soon, so add it now.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>