ASoC: fsl: imx-pcm-fiq: Use 'unsigned int' for period

Fix the following warning when building with W=1 option:

sound/soc/fsl/imx-pcm-fiq.c: In function 'snd_hrtimer_callback':
sound/soc/fsl/imx-pcm-fiq.c:76:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>