iwlwifi: name change from signal protection flag

This bit need to be set for both RTS/CTS or CTS-to-self protection, if
CTS-to-self is used, then uCode will  check the RXON_FLG_SELF_CTS_EN
status. Change the name from TX_CMD_FLG_RTS_CTS_MSK to TX_CMD_FLAG_PROT_REQUIRE_MSK
to match the behavior of the bit setting.

Also update comments to reflect which hardware uses which of the TX command
flags.

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