ALSA: snd-atmel-abdac: increase periods_min to 6 instead of 4

This patch increases periods_min to 6 from 4, this will remove any
hickups where the buffer is not filled fast enough from user space.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>