mtd: nand: sunxi: create sunxi_nfc_hw_ecc_enable()/disable() functions

The code used to enable/disable the hardware ECC engine is repeated in a
lot of places. Create two functions to avoid code duplication.

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