config: default duration to 150, not 0

[SirLouen]

Closes #272

The shipped dawn-config sets option duration '0' but the compiled-in default in uci_get_dawn_metric() is at 150 so a stock install runs with 0. That value is passed straight through as the 802.11k beacon-request measurement duration, and some client firmware refuses a 0-TU measurement window, returning an empty report with the Refused bit set:

`BEACON-RESP-RX e4:bc:aa:bb:db:99 137 04 000000...`. 

DAWN's hearing map then only ever contains the AP the client is already on, so better_ap_available() has nothing to compare and steering silently never fires. On my cluster, the phone was sitting at -80 dBm for >10 min with another AP of the same ESS 1 m away with more than -40 dBm.

Setting duration=150 immediately flipped the reports to mode 00 and steering worked. It's client-dependent (one Xiaomi phone refused, another tolerated 0), so it looks like a per-device quirk rather than a default.

Also, I would consider adding in src/utils/dawn_uci.c

if (ret.duration <= 0) {
    dawnlog_warning("duration=%d is invalid (0 disables 802.11k BEACON reports); using %d\n",
                    ret.duration, 150);
    ret.duration = 150;
}

To avoid duration to 0. Looking to hear the opinion