mtd: nand: sunxi: prevent a small memory leak

I moved the sanity check on ecc->size before the allocation so that we
don't leak memory on error.

Fixes: 05af074a4b73 ('mtd: nand: sunxi: check ecc->size values')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>