ASoC: ac97: add MODULE_ALIAS for the platform driver

So that modprobe can load the driver automatically when the platform device
appears.

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>