ASoC: omap McBSP: Clear rx_irq at probe time for OMAP4

On OMAP4 we have one interrupt line per McBSP port.
At probe time tx, and rx irq value will be -ENXIO,
and only the tx irq will get corrected.
In omap_mcbsp_request if the rx_irq is not 0 we proceed,
and try to request the interrupt, which will fail on
OMAP4 (rx_irq == -6).
To avoid this error, clear the rx_irq at probe time
on OMAP4.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Liam Girdwood <lrg@ti.com>