mtd: nand: sunxi: Actually use DMA for subpage reads

ecc->read_subpage is set to sunxi_nfc_hw_ecc_read_subpage_dma when
->dmac != NULL, but is then unconditionally overwritten in the common
init path.

Remove this extra assignment to allow usage of the DMA operation when
possible.

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