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: I33f81827697dca9d9a230cd9e219eaa5c9fd7d37
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


[sameert@codeaurora.org: manually solved merge conflicts]
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>