EDAC/armada_xp: 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>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jan Luebbe <jlu@pengutronix.de>
Link: https://lkml.kernel.org/r/20200311071728.4541-1-tiwai@suse.de