wl0_nband has wrong default value

state.js expects 2 (not 0) for 2.4 GHz band, otherwise it is not selectable in the web pages for the wireless part of the web UI (see web.c).

Searching for 'nband"' shows several files that expect "2" for 2.4GHz and "1" for 5GHz:
```c
if (nvram_match(strcat_r(prefix, "nband", tmp), "2"))	// 2.4G
...
else
```
and
```c
if (nvram_match(strcat_r(prefix, "nband", tmp), "2"))	// 2.4G
...
else if (nvram_match(strcat_r(prefix, "nband", tmp), "1"))	// 5G
```
with prefix being:
```c
snprintf(prefix, sizeof(prefix), "wl%d_", unit);
```