mtd: rawnand: au1550nd: Keep the driver compatible with on-die ECC engines

Following the introduction of the generic ECC engine infrastructure, it
was necessary to reorganize the code and move the ECC configuration in
the ->attach_chip() hook. Failing to do that properly lead to a first
series of fixes supposed to stabilize the situation. Unfortunately, this
only fixed the use of software ECC engines, preventing any other kind of
engine to be used, including on-die ones.

It is now time to (finally) fix the situation by ensuring that we still
provide a default (eg. software ECC) but will still support different
ECC engines such as on-die ECC engines if properly described in the
device tree.

There are no changes needed on the core side in order to do this, but we
just need to leverage the logic there which allows:
1- a subsystem default (set to Host engines in the raw NAND world)
2- a driver specific default (here set to software ECC engines)
3- any type of engine requested by the user (ie. described in the DT)

As the raw NAND subsystem has not yet been fully converted to the ECC
engine infrastructure, in order to provide a default ECC engine for this
driver we need to set chip->ecc.engine_type *before* calling
nand_scan(). During the initialization step, the core will consider this
entry as the default engine for this driver. This value may of course
be overloaded by the user if the usual DT properties are provided.

Fixes: dbffc8cc ("mtd: rawnand: au1550: Move the ECC initialization to ->attach_chip()")
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210928222258.199726-3-miquel.raynal@bootlin.com