mtd: nand: sunxi: make OOB retrieval optional

sunxi_nfc_hw_ecc_read_chunk() always retrieves the ECC and protected free
bytes, no matter if the user really asked for it or not. This can take a
non negligible amount of time, especially on NAND chips exposing large OOB
areas (> 1KB). Make it optional.

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