mtd: oxnas_nand: Allocating more than necessary in probe()

We only need to allocate sizeof(struct oxnas_nand_ctrl) which is 192
bytes and not sizeof(struct nand_chip) which is a much larger 3056
bytes.

Fixes: 66859249 ("mtd: nand: Add OX820 NAND Support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>