mtd: nand: ecc: Provide a helper to retrieve a pilelined engine device

In a pipelined engine situation, we might either have the host which
internally has support for error correction, or have it using an
external hardware block for this purpose. In the former case, the host
is also the ECC engine. In the latter case, it is not. In order to get
the right pointers on the right devices (for example: in order to devm_*
allocate variables), let's introduce this helper which can safely be
called by pipelined ECC engines in order to retrieve the right device
structure.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-16-miquel.raynal@bootlin.com