ALSA: snd-atmel-ac97c: set correct size for buffer hardware parameter

This patch will set a proper maximum bytes for the buffer, which is:
channels * bytes per sample * maximum periods * maximum bytes per period.

It also sets the minimum periods to 6, a value chosen from testing, with
a minimum of 6 periods the system has good time to fill in new audio
data without skipping.

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