mtd: rawnand: sunxi: introduce ecc_mode_mask in sunxi_nfc_caps

mtd: rawnand: sunxi: introduce ecc_mode_mask in sunxi_nfc_caps

The H6/H616 ECC_MODE field is not at the same offset, and has not the
same size.
So move the mask into sunxi_nfc_caps.

Also, introduce a non compile-time field_prep() because FIELD_PREP()
doesn't work with non compile-time constant.

No functional change.

Link: https://lore.kernel.org/all/cover.1761588465.git.geert+renesas@glider.be
Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>