iwlegacy: 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.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>