wifimngr: print backhaul stats on ubus call

Print the values from wifi_sta_stats struct into console on the following ubus call (station):

ubus call wifi.backhaul.wlanX stats

@janusz @Marek @bartlomiej.grzeskowiak Please review

Signed-off-by: Grzegorz Sluja grzegorz.sluja@iopsys.eu