ftl: use default_flash_is_zero() to check if pages are erased

source/ftl/dev.c

@@ -69,6 +69,7 @@ static struct flash_ops ftl_flash_ops = {
 	.write = ftl_flash_write,
 	/* TODO: implement default function to perform a copy. */
 	.copy = NULL,
+	.is_erased = default_flash_is_zero,
 	.erase = ftl_flash_erase,
 	.sync = ftl_flash_sync,
 };