iwlwifi: Fix issue on file transfer stalled in HT mode

Turn on RTS/CTS for HT to prevent uCode TX fifo underrun

This is fix for
http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2103

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Tested-by: Jiajia Zheng <jiajia.zheng@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>