iwlwifi: fixes RTS / CTS support

This patch fixes the RTS / CTS support in iwlwifi. 5000 will send CTS to
self when allowed by spec, 4965 will send RTS or CTS to self according to
mac80211 request.

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