ASoC: pcm: Fix (again) possible buffer overflow in dpcm state sysfs output

This is re-applying the fix that went into 5.6 (commit 6c89ffea

)
as the changes were wiped out after merging the other code
refactoring.  Basically the same changes, just replacing the
suspicious calls of snprintf() with scnprintf().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200310163625.10838-1-tiwai@suse.de


Signed-off-by: Mark Brown <broonie@kernel.org>