mtd: spinand: set bitflip_threshold to 75% of ECC strength

mtd: spinand: set bitflip_threshold to 75% of ECC strength

Reporting an unclean read from SPI-NAND only when the maximum number
of correctable bitflip errors has been hit seems a bit late.
UBI LEB scrubbing, which depends on the lower MTD device reporting
correctable bitflips, then only kicks in when it's almost too late.

Set bitflip_threshold to 75% of the ECC strength, which is also the
default for raw NAND.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/2117e387260b0a96f95b8e1652ff79e0e2d71d53.1723427450.git.daniel@makrotopia.org