fastboot: Proper download gadget unregister and clear when cable is missing

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>

common/cmd_fastboot.c

@@ -23,6 +23,8 @@ static int do_fastboot(cmd_tbl_t *cmdtp, int flag, int argc, char *const argv[])
 	if (!g_dnl_board_usb_cable_connected()) {
 		puts("\rUSB cable not detected.\n" \
 		     "Command exit.\n");
+		g_dnl_unregister();
+		g_dnl_clear_detach();
 		return CMD_RET_FAILURE;
 	}