spi: xilinx: Handle errors from platform_get_irq()

The Xilinx SPI driver can operate without an IRQ, but not every error
returned by platform_get_irq() means that no IRQ was specified. It will
also return an error if the IRQ specification is invalid or the IRQ
provider is not yet available (EPROBE_DEFER).

So instead of ignoring all errors only ignore ENXIO, which means no IRQ was
specified, and propagate all other errors to device driver core.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>