mtd: rawnand: sunxi: make the code mode self-explanatory

mtd: rawnand: sunxi: make the code mode self-explanatory

In sunxi_nfc_hw_ecc_{read,write}_chunk(), the ECC step was force to 0,
the reason is not trivial to get when reading the code.

The explanation is that, from the NAND flash controller perspective, we
are indeed at step 0 for user data length and ECC errors.

Just add a const value with an explanation to clarify things.

Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Reviewed-By: Michael Trimarchi <michael@amarulasolutions.com>
Acked-by: Andre Przywara <andre.przywara@arm.com>