mtd: sh_flctl: Expand the READID command to 8 bytes

The nand base code wants to read out 8 bytes in the READID command.
Reflect this in the driver code.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>