mtd: nand: sunxi: fix sunxi_nand_chips_cleanup()

The sunxi_nand_chips_cleanup() function is missing a call to list_del()
which generates a double free error.

Reported-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: <stable@vger.kernel.org> # 3.19+
Fixes: 1fef62c1

 ("mtd: nand: add sunxi NAND flash controller support")
Tested-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>