Fix the scan handling for 11b->11g upgrades in a world where, well,
it's not just 11b/11g.

The following was happening, and it's quite .. annoyingly grr-y.

* create vap, setup wpa_supplicant with no bgscanning, etc - there's
  no call to ieee80211_media_change, so vap->iv_des_mode is
  IEEE80211_MODE_AUTO;
* do ifconfig wlan0 scan - same thing, media_change doesn't get called,
  iv_des_mode stays as auto.
* But then, run wpa_cli and do 'scan' - it'll do a media change.
* if you're on 11ng, vap->iv_des_mode gets changed to IEEE80211_MODE_11NG
* Then makescanlist() is called.  There's a block of code that gets
  called if iv_des_mode != IEEE80211_MODE_AUTO, and it does this:

		if (vap->iv_des_mode != IEEE80211_MODE_11G ||
			    mode != IEEE80211_MODE_11B)
			continue;
		mode = IEEE80211_MODE_11G;	/* upgrade */

* .. now, iv_des_mode is not IEEE80211_MODE_11G, so it always runs
  'continue'
* .. and thus the scan list stays empty and no further channel
  scans occur. Ever.(1)

If you then disassociate and try associating to something, your
scan table has likely been purged / aged out and you'll never
see anything in the scan list.

(1) You need to do 'ifconfig wlan0 mode auto' or just destroy/re-create
    the VAP to get working wireless again.

Tested:

* iwn(4) - intel 5300 wifi; STA mode; using wpa_supplicant; bgscan
  enabled -and- wpa_supplicant scanning.

Thanks to:

* Everyone who kept poking me about this and wondering why the hell
  their wifi would eventually stop seeing scan lists.  Grr.
  I eventually snapped this evening and dug back into this code.