iwl3945: Fix rate scale for B-mode connection

This patch make sure we use only CCK rate for B only network.
For 3945 driver, it sets REPLY_RATE_SCALE command every time
we connect to a new network. In this command we set for every
rate the number of try and next rate. The problem mac80211 reports
same mode for both B and G mode as IEEE80211_BAND_2GHZ which will cause
using invalid rate other than CCK in B only network. THis patch
on association will examine sta valid rate on association, if no
OFDM rate in valid available rates it considers it as B only mode so
we can set REPLY_RATE_SCALE command with valid B only network and only
choose CCK rate in rate scaling.

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>