ASoC: atmel: Fix unlocked snd_pcm_stop() call

commit 571185717f8d7f2a088a7ac38d94a9ad5fd9da5c upstream.

snd_pcm_stop() must be called in the PCM substream lock context.

Acked-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>