dcd/samx7x: acknowledge the stale TXINI before deferring a ZLP

TXINI is sticky, so every bank freed during a DMA transfer leaves it set and the deferred ZLP handler fired at once, on whatever bank state happened to exist. Acknowledge it first, then send the ZLP right away when NBUSYBK shows no busy bank and from the next real TXINI otherwise. This holds for FIFO endpoints too, so the EP_DMA_SUPPORT gate goes away again