spi/imx: correct the test on platform_get_irq() return value

The test "if (spi_imx->irq <= 0)" is not testing the IRQ value, but
the return value of platform_get_irq().  As platform_get_irq() can
return an error (-ENXIO) or the IRQ value it found, the test should be
"if (spi_imx->irq < 0)"

[grant.likely: Note: In general, Linux irq number 0 should also mean
no irq, but arm still allows devices to be assigned 0, and the imx
platform uses 0 for one of the spi devices, so this patch is needed
for the device to work]

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>