mtd: rawnand: stm32_fmc2: fix broken ECC

Since commit d7157ff4

 ("mtd: rawnand: Use the ECC framework user
input parsing bits"), ECC are broken in FMC2 driver in case of
nand-ecc-step-size and nand-ecc-strength are not set in the device tree.
To avoid this issue, the default settings are now set in
stm32_fmc2_nfc_attach_chip function.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Fixes: d7157ff4

 ("mtd: rawnand: Use the ECC framework user input parsing bits")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1604064819-26861-1-git-send-email-christophe.kerello@st.com