iwlwifi: mvm: cleanup iwl_mvm_tx_skb_non_sta

Make the coupling of station id and queue id clear. Group code
together. Remove outdated comment. Never use an undefined hw
queue as given from mac80211.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>