dmi.c: Ensure g_has_dmi_support is default on shutdown

Ensure the g_has_dmi_support variable has the default state
of false after the life-time has expired.

BUG=none
TEST=builds

Change-Id: I0674950304736e53d014117d287682a4f6349879
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/62996
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Nikolai Artemiev <nartemiev@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

dmi.c

@@ -386,6 +386,7 @@ static int dmi_shutdown(void *data)
 		free(dmi_strings[i].value);
 		dmi_strings[i].value = NULL;
 	}
+	g_has_dmi_support = false;
 	return 0;
 }