iwlwifi: clear rate control flags on non-HT packet

Clear the flags (most importantly, the IEEE80211_TX_RC_MCS flag)
when sending a non-HT packet so that the rate index can be properly treated.
This fixes the reporting of legacy rates in wireless-extensions for packets
sent after an HT packet.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>