spl: fit: nand: fix fit loading in case of bad blocks

The offset at which the image to be loaded from NAND is located is
retrieved from the itb header. The presence of bad blocks in the area
of the NAND where the itb image is located could invalidate the offset
which must therefore be adjusted taking into account the state of the
sectors concerned.

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