nl80211: support vendor-specific events

In addition to vendor-specific commands, also support vendor-specific
events. These must be registered with cfg80211 before they can be used.
They're also advertised in nl80211 in the wiphy information so that
userspace knows can be expected. The events themselves are sent on a
new multicast group called "vendor".

Change-Id: I184aaa9f9e8461aee572f7d0a35916cd668c2218
CRs-fixed: 576020
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: 567ffc3509b2d3f965a49a18631d3da7f9a96d4f
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/linville/wireless-next.git
Signed-off-by: Leo Chang <leochang@codeaurora.org>