iwlwifi: mvm: change iwl_mvm_get_key_sta_id() to return the station

The code in iwl_mvm_update_tkip_key() is now pretty much duplicated
with the code in iwl_mvm_get_key_sta_id() doing the station ID lookup
again after it was already done. Change iwl_mvm_get_key_sta_id() to
iwl_mvm_get_key_sta(), returning the mvm_sta pointer, to allow that
duplicate code to be removed.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>