ALSA: sound/atmel/ac97c.c: Convert to module_platform_driver

This reduces some boilerplate code.

Signed-off-by: Alexander Stein <alexanders83@web.de>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>