spi: cadence_qspi: Fix checking return value of fdt_first_subnode()

fdt_first_subnode() returns -FDT_ERR_NOTFOUND if no subnode found.
0 is supposed to be a valid offset returns from fdt_first_subnode().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

drivers/spi/cadence_qspi.c

@@ -297,7 +297,7 @@ static int cadence_spi_ofdata_to_platdata(struct udevice *bus)
 
 	/* All other paramters are embedded in the child node */
 	subnode = fdt_first_subnode(blob, node);
-	if (!subnode) {
+	if (subnode < 0) {
 		printf("Error: subnode with SPI flash config missing!\n");
 		return -ENODEV;
 	}