spi: spi-imx: Propagate the real error code on platform_get_irq() failure

No need to return a 'fake' return value on platform_get_irq() failure.

Just return the error code itself instead.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>