mt76: mt7915: only modify tx buffer list after allocating tx token id

Modifying the tx buffer list too early can leak DMA mappings

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210216135119.23809-2-nbd@nbd.name