nl80211: Support ACS offload feature

Added support for offloading ACS functionality to host driver.
A new feature flag NL80211_FEATURE_AP_ACS_OFFLOAD is added to support
ACS (Automatic Channel Selection) offload to driver. This allows driver
to advertise its ACS capability.
When hostapd starts, if channel is set to 0 hostapd will check if driver
has the capability of doing ACS. If driver has such capability then the
channel selection is done in driver. Meanwhile kernel skips validation
of channel information since it is null when initially passed by hostapd.
After channel is selected by driver, a channel change event is sent from
driver to kernel. Kernel updates channel information it maintains using
the new channel.

CRs-fixed: 714883
Change-Id: I324e87e674aefcc6737ff571a543f4988419bef3
Signed-off-by: Peng Xu <pxu@codeaurora.org>