ALSA: oxfw: fix NULL pointer dereference to unused stream structure

ALSA oxfw driver supports Griffin FireWave. This device supports one
isochronous stream for PCM playback. Current driver executes code to
wait event for stream structure of opposite direction. This causes
NULL pointer dereference.

This commit fixes the bug.

Fixes: ac5d7786

 ("ALSA: oxfw: support AMDTP domain")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>