ASoC: atmel-ssc: make it buildable on other architectures

Not very useful on non AT91/AVR32 platforms but it provides
more build coverage and prepares for ARM multiplatform.

Also fixes a truncated warning that would come when
building on a 64-bit arch.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>