mt76: dma: use kzalloc instead of devm_kzalloc for txwi

[ Upstream commit 402e0109 ]

dma unmap is already needed for cleanup anyway, so we don't need the extra
tracking and can save a bit of memory here

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Stable-dep-of: 31762059 ("wifi: mt76: mt7603: improve stuck beacon handling")
Signed-off-by: Sasha Levin <sashal@kernel.org>