mtd: gpmi: remove the nand_scan()

mtd: gpmi: remove the nand_scan()

In order to make the nand_scan() work, the current code uses the hack code
to init the @nand_chip->ecc.size and the @nand_chip->ecc.strength. and
re-init some the ECC info in the gpmi_pre_bbt_scan().
This code is really a little ugly.

The patch does following changes:
  (1) Use the nand_scan_ident()/nand_scan_tail() to replace the nand_scan().

  (2) Init all the necessary values in the gpmi_init_last()
      before we call the nand_scan_tail().

  (3) remove the code setting the ECC info, let the mtd layer to do the
      real job.

  (4) remove the gpmi_scan_bbt(). we do not need this function any more.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>