dm: spi: Correct handling of SPI chip selects in sandbox

This code was not updated when the chip select handling was adjusted. Fix
it to call the correct function.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Heiko Schocher <hs@denx.de>
diff --git a/drivers/mtd/spi/sandbox.c b/drivers/mtd/spi/sandbox.c
index 1cf2f98..fecf6d4 100644
--- a/drivers/mtd/spi/sandbox.c
+++ b/drivers/mtd/spi/sandbox.c
@@ -602,7 +602,7 @@
 		       spec, ret);
 		return ret;
 	}
-	ret = device_find_child_by_seq(bus, cs, true, &slave);
+	ret = spi_find_chip_select(bus, cs, &slave);
 	if (!ret) {
 		printf("Chip select %d already exists for spec '%s'\n", cs,
 		       spec);