iwlwifi: use iwl_find_station less

Since we now store the station ID in each station
struct, many places need not look at the station
table any more since they can just pull the station
ID out of the struct. Remove iwl_get_sta_id() and
use iwl_sta_id() instead as appropriate.

This reduces the amount of code needed to find the
right station significantly, and works since
mac80211 passes the station only after it has been
fully initialised, ie. even if TX races with
station addition it will only be passed to TX once
the addition is complete.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>