flash/nand: review NAND driver interface

From struct nand_flash_controller :
- remove unused field register_commands;
- remove field controller_ready, exported but
  never referenced.
Remove dead code pointed by controller_ready.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>

src/flash/nand/s3c2440.c

@@ -166,6 +166,5 @@ struct nand_flash_controller s3c2440_nand_controller = {
 		.read_page = s3c24xx_read_page,
 		.write_block_data = &s3c2440_write_block_data,
 		.read_block_data = &s3c2440_read_block_data,
-		.controller_ready = &s3c24xx_controller_ready,
 		.nand_ready = &s3c2440_nand_ready,
 	};