iwlwifi: Fix throughput stall issue in HT mode for 5000

Similar to 6000 and 1000 series, RTS/CTS is the recommended protection
mechanism for 5000 series in HT mode based on the HW design.

Using RTS/CTS will better protect the inner exchange from interference,
especially in highly-congested environment, it also prevent uCode encounter
TX FIFO underrun and other HT mode related performance issues.

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