mtd: nand: fix the wrong mtd->type for nand chip

Current code sets the mtd->type with MTD_NANDFLASH for both
SLC and MLC. So the jffs2 may supports the MLC nand, but in actually,
the jffs2 should not support the MLC.

This patch uses the nand_is_slc() to check the nand cell type,
and set the mtd->type with the right nand type.

After this patch, the jffs2 only supports the SLC nand.

The side-effect of this patch:
  Before this patch, the ioctl(MEMGETINFO) can only return with the
  MTD_NANDFLASH; but after this patch, the ioctl(MEMGETINFO) will
  return with the MTD_NANDFLASH for SLC, and MTD_MLCNANDFLASH for MLC.

  So the user applictions(such as mtd-utils) should also changes a little
  for this.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>