ALSA: pcm: Fix document for snd_pcm_stop_xrun()

Fix a copy & paste error:
 Warning(sound/core/pcm_native.c:1112): Excess function parameter 'state' description in 'snd_pcm_stop_xrun'

The state argument was dropped from snd_pcm_stop_xrun().

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>