iwlwifi: take a fresh set of supported rates at each cycle

This patch fixes regression in iwlwifi IBSS rate scaling caused by the
patch named "mac80211: eliminate IBSS warning in rate_lowest_index()"
by Vladimir Koutny <vlado@work.ksp.sk>.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>