mtd: nand: sunxi: retrieve corrected OOB bytes

The ECC engine is protecting a few OOB bytes. Retrieve them from the
USER_DATA register instead of reading them in raw mode (ie without the ECC
protection).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>