ALSA: fireworks: Use safer way to arrange ring buffer pointer

To reverse a pointer for the ring buffer, subtraction by buffer
size is better than assignment to the beginning of the buffer.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --cc sound/firewire/fireworks/fireworks_hwdep.c
Simple merge

diff --cc sound/firewire/fireworks/fireworks_transaction.c
Simple merge