dmaengine: imx-sdma: Return a proper error code in platform_get_irq()

There is no need to return a 'fake' value upon platform_get_irq() failure.

Just propagate the real error instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>