drivers/mtd/mtd_cfi: Add MTDIOC_ERASESTATE

We need to give a default erase value when we register cfi flash for mtd devices
he pseudo-code is as follows：
register_cfi_driver(xxx)
find_mtddriver("xxx", &inode);
mtdconfig_register(inode->u.i_mtd);

https://github.com/Zhangshoukui/nuttx/blob/master/drivers/mtd/mtd_config.c#L1775C28-L1775C45

Signed-off-by: zhangshoukui <zhangshoukui@xiaomi.com>

drivers/mtd/mtd_cfi.c

@@ -235,6 +235,13 @@ static int cfi_mtd_ioctl(FAR struct mtd_dev_s *dev, int cmd,
         }
         break;
 
+      case MTDIOC_ERASESTATE:
+        {
+          FAR uint8_t *result = (FAR uint8_t *)arg;
+          *result = 0xff;
+          break;
+        }
+
       default:
         ret = -ENOTTY;
         break;