dfu: avoid memory leak

When dfu_fill_entity fail, need to free dfu to avoid memory leak.

Reported by Coverity:
"
Resource leak (RESOURCE_LEAK)
leaked_storage: Variable dfu going out of scope leaks the storage
it points to.
"

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: "Łukasz Majewski" <l.majewski@samsung.com>
Cc: Marek Vasut <marex@denx.de>