cmd_nand: fix a memory leak in nand_dump function

If datbuf = memalign(ARCH_DMA_MINALIGN, nand->writesize);
succeeds and
  oobbuf = memalign(ARCH_DMA_MINALIGN, nand->oobsize);
fails, nand_dump function should free databuf.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>