dfu: Fix handling of UBI partitions in MTD backend

For UBI partitions ("partubi" in dfu_alt_info), dfu_fill_entity_mtd sets
the mtd.ubi flag; however other functions incorrectly check for nand.ubi
instead. Fix this by checking for the correct flag.

Signed-off-by: Guillermo Rodriguez <guille.rodriguez@gmail.com>
Cc: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>