iwlwifi: fix rs_get_rate WARN_ON()

In ieee80211_sta structure there is u64 supp_rates[IEEE80211_NUM_BANDS]
this is filled with all support rate from assoc_resp.  If we associate
with G-band AP only supp_rates of G-band will be set the other band
supp_rates will be set to 0. If the user type this command
this will cause mac80211 to set to new channel, mac80211
does not disassociate in setting new channel, so the active
band is now A-band. then in handling the new essid mac80211 will
kick in the assoc steps which involve sending disassociation frame.
in this mac80211 will WARN_ON sta->supp_rates[A_BAND] == 0.

This fixes:
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1822
http://www.kerneloops.org/searchweek.php?search=rs_get_rate

Signed-off-by: mohamed abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>