ALSA: firewire: fix comparison to bool warning

Fix the following coccicheck warning:

./sound/firewire/amdtp-stream.h:273:6-19: WARNING: Comparison to bool

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Link: https://lore.kernel.org/r/1604765611-8209-1-git-send-email-kaixuxia@tencent.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>