mtd: spinand: macronix: Fix ECC Status Read

The datasheet specifies the upper four bits are reserved.
Testing on real hardware shows that these bits can indeed be nonzero.

Signed-off-by: Emil Lenngren <emil.lenngren@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>