ASoC: kirkwood: fix compilation warning in kirkwood_dma_open

writel() supposes the first argument of type unsigned int. This fix the
warning:

sound/soc/kirkwood/kirkwood-dma.c: In function 'kirkwood_dma_open':
sound/soc/kirkwood/kirkwood-dma.c:164:3: warning: large integer implicitly
truncated to unsigned type [-Woverflow]

Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>