mtd: omap2: mtd split nand_scan in ident and tail

nand_scan calls nand_scan_tail and here we got a ecc.layout and calculate
oobavail for this layout. After calling nand_scan, we change the layout pointer
if OMAP_ECC_HAMMING_CODE_HW_ROMCODE is set. This results in not calcluated
oobavail. Mountig as jffs2 is not possible.

To fix that nand_scan has to split up in nand_scan_ident and nand_scan_tail
setting ecc.layout between these calls. So nand_scan_tail calculates oobvail
for the used layout. This is also done in serveral other platforms.

Signed-off-by: Jan Weitzel <j.weitzel@phytec.de>
Reviewed-by: Vimal Singh <vimal.newwork@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>