ASoC: omap-mcbsp: Fix 6pin mux configuration

commit d0db84e713eaaccea2a435e1625fb3150b335f4a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>