mtd: rawnand: micron: Make ECC activation stateful

We currently don't store the on-die ECC state (enabled/disabled) which
might force us to re-disable the engine even if it's already been
disabled after we've read the page in raw mode to count the actual
number of bitflips.

Add an "enabled" field to struct micron_on_die_ecc to keep track of the
ECC state.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>