[ALSA] Fix bug in snd-usb-usx2y's usX2Y_pcms_lock_check()

Fix bug in snd-usb-usx2y's usX2Y_pcms_lock_check()
substream can be NULL......
in mainline, bug was introduced by:
2006-06-22  [ALSA] Add O_APPEND flag support to PCM

Signed-off-by: Karsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>