fix: nand: Replace hardcoded page chunk size with calculated one

Replace the hardcoded value of page chink with value that
depends on flash page size and ECC strength.
This fixes nand access errors for 2K page flashes with 8-bit ECC.
Move the initial flash commannd function assignment past the ECC
structures initialization for eliminating usage of hardcoded page
chunk size value.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>