ASoC: omap-mcbsp: Fix 6pin mux configuration

The check for the mux_signal callback was wrong which prevents us to
configure the 6pin port's FSR/CLKR signal mux.

Reported-by: CF Adad <cfadad@rocketmail.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org (3.4+)

sound/soc/omap/mcbsp.c

@@ -745,7 +745,7 @@ int omap_mcbsp_6pin_src_mux(struct omap_mcbsp *mcbsp, u8 mux)
 {
 	const char *signal, *src;
 
-	if (mcbsp->pdata->mux_signal)
+	if (!mcbsp->pdata->mux_signal)
 		return -EINVAL;
 
 	switch (mux) {