D-Bus: Fix P2P Group PSK property getter

D-Bus: Fix P2P Group PSK property getter

This was returning a byte array of the pointer to the PSK, not the
actual PSK, due to incorrect use of
wpas_dbus_simple_array_property_getter(). In addition, there is no need
to limit this property based on the role of the device in the group, so
return the PSK if it is available (which it will be for both GO and P2P
Client roles).

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