iwlwifi: queue: don't crash if txq->entries is NULL

The code was really awkward, we would first dereference
txq->entries when calling iwl_txq_genX_tfd_unmap and then
we would check that txq->entries is non-NULL.
Fix that by exiting if txq->entries is NULL.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210115130252.173359fc236d.I75c7c2397d20df8d7fbc24cb16a5232d5c551889@changeid