iwlwifi: fix rate control regression

Since driver does not use control.rates[0].count, we have never set that
variable. But currently, after rate control API rewrite, this is required
by mac80211. Otherwise legacy rates control does not work and we transmit
always at 1Mbit/s on pre 11n networks.

[same fix as for iwlegacy, thanks Stanislaw!]

Signed-off-by: Moshe Benji <Moshe.Benji@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>