MFC r318005:

[iwm] Add basic powermanagement support via ifconfig wlan0 powersave.

* The DEVICE_POWER_FLAGS_CAM_MSK flag was removed in the upstream iwlwifi
in Linux commit ceef91c89480dd18bb3ac51e91280a233d0ca41f.

* Add sc_ps_disabled flag to struct iwm_softc, which corresponds to
mvm->ps_disabled in struct iwl_mvm in Linux iwlwifi.

* Adds a hw.iwm.power_scheme tunable which corresponds to the power_scheme
module parameter in Linux iwlwifi. Set this to 1 for completely
disabling power management, 2 (default) for balanced powermanagement,
and 3 for lowerpower mode (which does dtim period skipping).

* Imports the constants.h file from iwlwifi as if_iwm_constants.h.

* This doesn't allow changing the powermanagement setting while connected,
also one can only choose between enabled and disabled powersaving with
ifconfig (so switching between balanced and low-power mode requires
rebooting to change the tunable).

* After any changes to powermanagement (i.e. "ifconfig wlan0 powersave" to
enable powermanagement, or "ifconfig wlan0 -powersave" for disabling
powermanagement), one has to disconnect and reconnect to the accespoint
for the change to take effect.