mtd: spinand: Fix comment

This is a copy paste error, checking the ECC status finishes a page read
here, not a page write.

Fixes: 945845b5 ("mtd: spinand: Instantiate a SPI-NAND on-die ECC engine")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210629195157.567828-1-miquel.raynal@bootlin.com