mtd: rawnand: stm32_fmc2: fix ECC overwrite

commit 811c0da4542df3c065f6cb843ced68780e27bb44 upstream.

In case OOB write is requested during a data write, ECC is currently
lost. Avoid this issue by only writing in the free spare area.
This issue has been seen with a YAFFS2 file system.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Cc: stable@vger.kernel.org
Fixes: 2cd457f3 ("mtd: rawnand: stm32_fmc2: add STM32 FMC2 NAND flash controller driver")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>