imx: Change USB boot device type

The SPL SDP is configured as BOOT_DEVICE_BOARD, so when booting from
USB, change its type to BOOT_DEVICE_BOARD, so we can use SDP.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/spl.c b/arch/arm/mach-imx/spl.c
index 64ca296..e89e227 100644
--- a/arch/arm/mach-imx/spl.c
+++ b/arch/arm/mach-imx/spl.c
@@ -177,7 +177,7 @@
 	case QSPI_BOOT:
 		return BOOT_DEVICE_NOR;
 	case USB_BOOT:
-		return BOOT_DEVICE_USB;
+		return BOOT_DEVICE_BOARD;
 	default:
 		return BOOT_DEVICE_NONE;
 	}