spi: dw: Use generic function to read reg address

Using an fdt-specific function causes problems when compiled with a live
tree.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Tested-by Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/drivers/spi/designware_spi.c b/drivers/spi/designware_spi.c
index 32de33f..e8ba80e 100644
--- a/drivers/spi/designware_spi.c
+++ b/drivers/spi/designware_spi.c
@@ -160,6 +160,8 @@
 	struct dw_spi_platdata *plat = bus->platdata;
 
 	plat->regs = dev_read_addr_ptr(bus);
+	if (!plat->regs)
+		return -EINVAL;
 
 	/* Use 500KHz as a suitable default */
 	plat->frequency = dev_read_u32_default(bus, "spi-max-frequency",