mtd: nftl: reorganize operations in condition check

We need to compare ret variable for negative value. The current code
assigns the boolean to the ret and prints it wrongly in the warning
message.

Reported-by: Andrey Karpov <karpov@viva64.com>
Cc: Giel van Schijndel <me@mortis.eu>
Cc: Dimitri Gorokhovik <dimitri.gorokhovik@free.fr>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>