wireless: iwlwifi: Fix a double free in iwl_txq_dyn_alloc_dma

wireless: iwlwifi: Fix a double free in iwl_txq_dyn_alloc_dma

In iwl_txq_dyn_alloc_dma, txq->tfds is freed at first time by:
iwl_txq_alloc()->goto err_free_tfds->dma_free_coherent(). But
it forgot to set txq->tfds to NULL.

Then the txq->tfds is freed again in iwl_txq_dyn_alloc_dma by:
goto error->iwl_txq_gen2_free_memory()->dma_free_coherent().

My patch sets txq->tfds to NULL after the first free to avoid the
double free.

Fixes: 0cd1ad2d7fd41 ("iwlwifi: move all bus-independent TX functions to common code")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Link: https://lore.kernel.org/r/20210403054755.4781-1-lyl2019@mail.ustc.edu.cn
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>