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/lpc3180.c

@@ -1363,6 +1363,5 @@ struct nand_flash_controller lpc3180_nand_controller = {
 	.read_data = lpc3180_read_data,
 	.write_page = lpc3180_write_page,
 	.read_page = lpc3180_read_page,
-	.controller_ready = lpc3180_controller_ready,
 	.nand_ready = lpc3180_nand_ready,
 };