iwlwifi: name changes from "tx_power_channel_lmt" to "tx_power_device_lmt"

Changing the name from "tx_power_channel_lmt" to "tx_power_device_lmt";
to give idea that scope of limit is for overall device, not any
individual channels

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>