iwlwifi: allow user change protection mechanism for HT

Allow user to change protection mechanism for HT between RTS/CTS and
CTS-to-self through sysfs:

Show current protection mechanism for HT
   cat /sys/class/net/wlan0/device/rts_ht_protection

Change protection mechanism for HT (only allowed while not-associated)
CTS-to-self:
   echo 0 > /sys/class/net/wlan0/device/rts_ht_protection
RTS/CTS:
   echo 1 > /sys/class/net/wlan0/device/rts_ht_protection

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