spi-imx: correct check for platform_get_irq failing

platform_get_irq returns -ENXIO if there is no entry.  So ensure
return value is greater than zero instead of non-zero.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>