dmaengine: ppc4xx: Use scnprintf() for avoiding potential buffer overflow

Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200311071606.4485-1-tiwai@suse.de
Signed-off-by: Vinod Koul <vkoul@kernel.org>