OMAP: mcbsp: Fix error checks of rx_irq.

rx_irq is intialized via a call to platform_get_irq_byname(), which can
return a negative error value.  Also, zero is a valid irq.  Fix checks
that assumed non-zero values were the only valid rx_irq values.

Change-Id: I6be171eefa5923021e4adccf71f21baad3383554
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>