mfd: aat2870: Use scnprintf() for avoiding potential buffer overflow

There is still one call of sprintf() without checking the proper
buffer overflow in aat2870_dump_reg().  Replace it with scnprintf()
call for covering that.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>