prometheus-node-exporter-lua: fix bss identifier in hostapd_stations

prometheus-node-exporter-lua: fix bss identifier in hostapd_stations

We previously did not identify the correct BSS from the output of
`hostapd_cli -i <phy> status`, because when asked for a vif it will
always respond with information relevant to the whole phy.

The per vif settings will use an iterator and we now try to detect
the correct BSS from that output.

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>