ath10k: switch to ieee80211_tx_dequeue_ni

Since ath10k_mac_tx_push_txq() can be called from process context, we
must explicitly disable softirqs before the call into mac80211.

By calling ieee80211_tx_dequeue_ni() instead of ieee80211_tx_dequeue()
we make sure softirqs are always disabled even in the case when
ath10k_mac_tx_push_txq() is called from process context.

Calling ieee80211_tx_dequeue_ni() with softirq's already disabled
(e.g., from softirq context) should be safe as the local_bh_disable()
and local_bh_enable() functions (called from ieee80211_tx_dequeue_ni)
are fully reentrant.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>