hostapd: hostapd_set_psk_file: fix defaut value for mac

hostapd: hostapd_set_psk_file: fix defaut value for mac

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Bringing up of station vlan fails if the optional mac entry isn't set.
The default mac "00:00:00:00:00:00", which should match all stations,
is mistakenly set to the non used variable "isolate". This results in
a wrong formatted .psk file which has to be "vlan_id mac key".

fixes: 5aa2ddd0: hostapd: add support for wifi-station and wifi-vlan sections

Signed-off-by: Johann Neuhauser <johann@it-neuhauser.de>