Set WoWLAN triggers only if driver capabilities are known

Set WoWLAN triggers only if driver capabilities are known

Previously, wpas_set_wowlan_triggers() could have been called in
uninitialized wpa_driver_capa data if the driver interface did not
support reporting of capabilities. While this would not really happen
with a driver wrapper that implements set_wowlan() and as such, would
not cause any difference in practice, it is better to clean this up to
make the code path easier to understand for static analyzers.

Signed-off-by: Jouni Malinen <j@w1.fi>