spi/xilinx: signedness issue checking platform_get_irq()

In xilinx_spi_probe() we use xspi->irq to store negative error codes so
it has to be signed.  We weren't going to use the upper bit any way so
this is fine.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@linaro.org>