arm: stm32mp: Fix USB boot device report

In case the SoC reports the boot device type is USB, it means the SPL was
loaded via BootROM DFU mode. Currently the spl_boot_device() returns boot
device as USB host, change it to DFU instead, so the SPL can continue the
DFU boot and load U-Boot via DFU.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/arch/arm/mach-stm32mp/spl.c b/arch/arm/mach-stm32mp/spl.c
index 405eff6..51fe069 100644
--- a/arch/arm/mach-stm32mp/spl.c
+++ b/arch/arm/mach-stm32mp/spl.c
@@ -43,7 +43,7 @@
 	case BOOT_SERIAL_UART_8:
 		return BOOT_DEVICE_UART;
 	case BOOT_SERIAL_USB_OTG:
-		return BOOT_DEVICE_USB;
+		return BOOT_DEVICE_DFU;
 	case BOOT_FLASH_NAND_FMC:
 		return BOOT_DEVICE_NAND;
 	case BOOT_FLASH_NOR_QSPI: