mt76: mt7915: fix processing txfree events

In the MT7915 info, the fields for the WLAN index / queue overlap with the
token id, and the MT_TX_FREE_PAIR bit indicates, which one is present.

If MT_TX_FREE_PAIR is set, skip processing the token index, since the data
will not be valid.

This fixes accidentally freeing tokens which are still in use by the
hardware with a wrong station pointer.

Signed-off-by: Felix Fietkau <nbd@nbd.name>