Add an optional set_mode() driver_ops handler for setting mode before keys

A bug just got reported as a result of this for mac80211 drivers.

https://bugzilla.redhat.com/show_bug.cgi?id=459399

The basic problem is that since taking the device down clears the keys
from the driver on many mac80211-based cards, and since the mode gets
set _after_ the keys have been set in the driver, the keys get cleared
on a mode switch and the resulting association is wrong.  The report is
about ad-hoc mode specifically, but this could happen when switching
from adhoc back to managed mode.

src/drivers/driver_test.c

@@ -982,5 +982,6 @@ const struct wpa_driver_ops wpa_driver_test_ops = {
 	NULL /* update_ft_ies */,
 	NULL /* send_ft_action */,
 	wpa_driver_test_get_scan_results2,
-	NULL /* set_probe_req_ie */
+	NULL /* set_probe_req_ie */,
+	NULL /* set_mode */
 };