ramips: correct page read return value of the mt7621 nand driver

ramips: correct page read return value of the mt7621 nand driver

read_page() need to return maximum number of bitflips instead of the
accumulated number. Change takes from upstream mt7621 u-boot [1].

 * @read_page:  function to read a page according to the ECC generator
 *              requirements; returns maximum number of bitflips
 *              corrected in any single ECC step, -EIO hw error

[1] https://lore.kernel.org/all/cover.1653015383.git.weijie.gao@mediatek.com/

Signed-off-by: Shiji Yang <yangshiji66@qq.com>