ALSA: fireworks: fix an endianness bug for transaction length

Although the 't->length' is a big-endian value, it's used without any
conversion. This means that the driver always uses 'length' parameter.

Fixes: 555e8a8f("ALSA: fireworks: Add command/response functionality into hwdep interface")
Reported-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>