mtd: rawnand: gpio: Do not force a particular software ECC engine

Originally, commit d7157ff4 ("mtd: rawnand: Use the ECC framework
user input parsing bits") kind of broke the logic around the
initialization of several ECC engines.

Unfortunately, the fix (which indeed moved the ECC initialization to
the right place) did not take into account the fact that a different
ECC algorithm could have been used thanks to a DT property,
considering the "Hamming" algorithm entry a configuration while it was
only a default.

Add the necessary logic to be sure Hamming keeps being only a default.

Fixes: f6341f64 ("mtd: rawnand: gpio: Move the ECC initialization to ->attach_chip()")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201203190340.15522-4-miquel.raynal@bootlin.com