mtd: rawnand: mxc: Move the ECC engine initialization to the right place

No ECC initialization should happen during the host controller probe.

In fact, we need the probe function to call nand_scan() in order to:

- identify the device, its capabilities and constraints (nand_scan_ident())
- configure the ECC engine accordingly (->attach_chip())
- scan its content and prepare the core (nand_scan_tail())

Moving these lines to mxcnd_attach_chip() fixes a regression caused by
a previous commit supposed to clarify these steps.

When moving the ECC initialization from probe() to attach(), get rid
of the pdata usage to determine the engine type and let the core decide
instead.

Tested on a imx27-pdk board.

Fixes: d7157ff4 ("mtd: rawnand: Use the ECC framework user input parsing bits")
Reported-by: Fabio Estevam <festevam@gmail.com>
Co-developed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201016213613.1450-1-festevam@gmail.com