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>