spl: fit: improve spl_nand_fit_read(...) readability

Replacing the ret variable with err and handling first the error
condition about the value returned by the spl_nand_fit_read routine,
improves the code readability.
Furthermore, the 'else' int the 'else return ret' instruction was
useless.

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