ASoC: Fix trivial build regression in Kirkwood I2S

A fix merged in 3.1-rc2 introduced a small regression, this should get it
to build again.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>