wifi: mt76: route nullfunc frames to PSD/ALTX queue

wifi: mt76: route nullfunc frames to PSD/ALTX queue

ieee80211_is_data() returns true for nullfunc/QoS-nullfunc frames, so
they bypass the PSD queue routing and go through the regular VO data
queue. This means firmware processes them through the normal TID queue
instead of the ALTX queue, which doesn't guarantee immediate
transmission.

Use ieee80211_is_data_present() instead, which returns false for both
management frames and nullfunc/QoS-nullfunc (no payload), routing them
to MT_TXQ_PSD. Firmware maps PSD to the ALTX queue, which transmits
immediately without PS buffering.

This only affects frames from the mt76_tx() pending path. Regular
mac80211 TXQ scheduling is unchanged.

Link: https://patch.msgid.link/20260309060730.87840-9-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>