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+)