ALSA: atmel: Fix possible array overflow

The static checker found a possible array overflow in atmel/abdac.c:
  static checker warning: "sound/atmel/abdac.c:373 set_sample_rates()
        error: buffer overflow 'dac->rates' 6 <= 6"

This patch papers over the buggy point, by ensuring that dac->rates[]
update not overflowing the actual array size.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>