spl: spl_nand: Fix bad block handling in fitImage

If the fitImage has some bad block in fit image area, the
offset must be recalulcated. This should be done always.
After implementing it in mxs now is possible to call the function
even for that platform.

Cc: Fabio Estevam <festevam@gmail.com>
Tested-By: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>