wifi: mt76: dma: fix a regression in adding rx buffers

When adding WED support, mt76_dma_add_buf was accidentally changed to set
the skip_buf0 flag for tx buffers on the wrong queue descriptor entry.
Additionally, there is a rxwi leak when rx buffer allocation fails.

Fix this and make the code more readable by adding a separate function for
adding rx buffers.

Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Link: https://lore.kernel.org/r/CABXGCsMEnQd=gYKTd1knRsWuxCb=Etv5nAre%2BXJS_s5FgVteYA@mail.gmail.com/


Reported-by: Mike Lothian <mike@fireburn.co.uk>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216829


Reported-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/lkml/20230112171706.294550-1-angelogioacchino.delregno@collabora.com/


Fixes: cd372b8c ("wifi: mt76: add WED RX support to mt76_dma_{add,get}_buf")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230113105848.34642-3-nbd@nbd.name