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

sound/soc/fsl/imx-pcm-fiq.c

@@ -34,7 +34,7 @@
 #include "imx-ssi.h"
 
 struct imx_pcm_runtime_data {
-	int period;
+	unsigned int period;
 	int periods;
 	unsigned long offset;
 	unsigned long last_offset;