mtd: atmel_nand: Simplify error messages

The error messages when the ECC controller is misconfigured through the
device tree are very precise. As a result they can (and will) get
obsolete when new revisions of the controller appear.

Simplify them before adding the support for the new revision.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>