package: fix segfault of iwinfo.scanlist("radio0").

This is a bug revealed in r41830.

First, the static variable `char nif[IFNAMSIZ]` of nl80211_phy2ifname()
would be zeroed out if the argument is "wlan0" or the like.  This will
happen in the following call stack.

 nl80211_get_scanlist("radio0", buf, len);
   nl80211_phy2ifname("radio0")			// return static var nif with content "wlan0"
   nl80211_get_scanlist(nif, buf, len);		// tail call
     nl80211_get_mode(nif);
        nl80211_phy2ifname(nif);		// zero out nif

Later we try nl80211_ifadd("") which was supposed to create interface
"tmp.", but that won't happen because nl80211_msg() will put an invalid
ifidx 0 to the nlmsg.

Then iwinfo_ifup() and iwinfo_ifdown() would fail and happily
nl80211_get_scanlist() returned 0 and left *len undefined.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42151 3c298f89-4303-0410-b956-a3cf2f4a3e73