mwl8k: Fix a double Free in mwl8k_probe_hw

In mwl8k_probe_hw, hw->priv->txq is freed at the first time by
dma_free_coherent() in the call chain:
if(!priv->ap_fw)->mwl8k_init_txqs(hw)->mwl8k_txq_init(hw, i).

Then in err_free_queues of mwl8k_probe_hw, hw->priv->txq is freed
at the second time by mwl8k_txq_deinit(hw, i)->dma_free_coherent().

My patch set txq->txd to NULL after the first free to avoid the
double free.

Fixes: a66098da ("mwl8k: Marvell TOPDOG wireless driver")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210402182627.4256-1-lyl2019@mail.ustc.edu.cn