mac80211_hwsim: indicate support for powersave.

Without this, higher layers in the kernel will return an error
code when trying to set the power state because the driver
doesn't indicate power state support. This in turn causes VTS
(Android Vendor Test Suite) failures because the WiFi HAL can't
enable power saving mode.

Signed-off-by: Bjoern Johansson <bjoernj@google.com>
Signed-off-by: Lingfeng Yang <lfy@google.com>
Signed-off-by: Roman Kiryanov <rkir@google.com>
[johannes: remove remaining code, it was useless even as a skeleton
 since it didn't even have the right function arguments]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>