ASoC: fsl: imx-audmux: Fix sparse warning

Fix the following sparse warning:

sound/soc/fsl/imx-audmux.c:182:3: warning: symbol 'audmux_type' was not declared. Should it be static?
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/fsl/imx-audmux.c

@@ -176,7 +176,7 @@ static inline void audmux_debugfs_remove(void)
 }
 #endif
 
-enum imx_audmux_type {
+static enum imx_audmux_type {
 	IMX21_AUDMUX,
 	IMX31_AUDMUX,
 } audmux_type;