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>