[ARM] S3C: Fix S3C24XX build to not include s3c64xx IIS devices

Commit 52da219e added IIS
platform devices, but these do not build on s3c24xx systems
and the file depends on SND_S3C24XX_SOC, which is selected
for all S3C64XX/S3C24XX systems.

As a quick fix, make the dev-audio.o file depends on
SND_S3C64XX_SOC_I2S instead.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>