ASoC: kirkwood: Fix compile error due to patch 'add S/PDIF support'

This patch fixes the compilation error of kirkwood-i2s.c introduced
by the commit 75b9b65e

 'ASoC: kirkwood: add S/PDIF support'.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mark Brown <broonie@linaro.org>