iwlwifi: pcie: use seq_file for tx_queue debugfs file

On newer hardware, the tx_queue debugfs file would need to
allocate 37.5kib data since there are 512 queues, which is
too much. Rather than resorting to kludges like kvmalloc(),
use the seq_file API to print out the data.

While at it, also fix a NULL pointer dereference here, the
txq pointer from trans_pcie->txqs[] may be NULL if that
queue hasn't been allocated.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200417131727.491cf8224c49.I7f154d81e5becef3b5ff22d7c6e36170bde0d7d5@changeid