ALSA: oss: fix compile error when OSS_DEBUG is enabled

ALSA: oss: fix compile error when OSS_DEBUG is enabled

Fix compile error when OSS_DEBUG is enabled:
    sound/core/oss/pcm_oss.c: In function 'snd_pcm_oss_set_trigger':
    sound/core/oss/pcm_oss.c:2055:10: error: 'substream' undeclared (first
    use in this function); did you mean 'csubstream'?
      pcm_dbg(substream->pcm, "pcm_oss: trigger = 0x%x\n", trigger);
              ^

Fixes: 27ea358d6dfe ("ALSA: oss: Use standard printk helpers")
Signed-off-by: Bixuan Cui <cuibixuan@linux.alibaba.com>
Link: https://lore.kernel.org/r/1638349134-110369-1-git-send-email-cuibixuan@linux.alibaba.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>