wireless: Fix WARN_ON() with ieee802.11b

When the driver registers a IEEE80211_BAND_2GHZ band,
it can either be 802.11b or 802.11g. But when 802.11b rates
are registered "want" will be 3 (since 4 rates are being registered,
and each of those 4 rates will decrease "want").
Since this is a correct situation, there is no need to trigger
a WARN_ON() for this.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>