nl80211: Support ACS offload feature flag and attribute

Added support for offloading ACS functionality to host driver.
A new feature flag NL80211_FEATURE_AP_ACS_OFFLOAD and a new attribute
NL80211_ATTR_ACS_OFFLOAD are 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 hostapd set the attribut NL80211_ATTR_ACS_OFFLOAD to indicate to
kernel that 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: I195b0978031d7fad4f93688a0f919fd9a4d54193
Signed-off-by: Peng Xu <pxu@codeaurora.org>