mt76: dma: fix buffer unmap with non-linear skbs

mt76 dma layer is supposed to unmap skb data buffers while keep txwi
mapped on hw dma ring. At the moment mt76 wrongly unmap txwi or does
not unmap data fragments in even positions for non-linear skbs. This
issue may result in hw hangs with A-MSDU if the system relies on IOMMU
or SWIOTLB. Fix this behaviour properly unmapping data fragments on
non-linear skbs.

Fixes: 17f1de56

 ("mt76: add common code shared between multiple chipsets")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>