IWM_MAX_CMD_PAYLOAD_SIZE = 4088, hdr_wide = 8, hw TB cap = 4092
  paylen     totlen  enc.len(bits)     dec.length verdict
    4084       4092          65472           4092  ok
    4085       4093          65488           4093  EXCEEDS HW TB CAP (4K-4)
    4086       4094          65504           4094  EXCEEDS HW TB CAP (4K-4)
    4087       4095          65520           4095  EXCEEDS HW TB CAP (4K-4)
    4088       4096              0              0  EXCEEDS HW TB CAP (4K-4)
    4088       4096              0              0  EXCEEDS HW TB CAP (4K-4)

At paylen=4088 (the maximum iwm_send_cmd() accepts) with a wide header,
the firmware-visible TB0 length is 0 -- the command payload is invisible to fw.