wcn36xx: Release DMA channel descriptor allocations

When unloading the driver we are not releasing the DMA descriptors which we
previously allocated.

Fixes: 8e84c258 ("wcn36xx: mac80211 driver for Qualcomm WCN3660/WCN3680 hardware")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211105122152.1580542-3-bryan.odonoghue@linaro.org