spl: fit: fail fit loading in case of FDT appending error

If uboot does not embed its device tree and the FIT loading function
returns error in case of failure in the FDT append, the redundant itb
image could be loaded.

cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>