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

drivers/spi/spi-xilinx.c

@@ -82,7 +82,7 @@ struct xilinx_spi {
 	struct completion done;
 	void __iomem	*regs;	/* virt. address of the control registers */
 
-	u32		irq;
+	int		irq;
 
 	u8 *rx_ptr;		/* pointer in the Tx buffer */
 	const u8 *tx_ptr;	/* pointer in the Rx buffer */