nl80211: vendor command support

Add support for vendor-specific commands to nl80211. This is
intended to be used for really vendor-specific functionality
that can't be implemented in a generic fashion for any reason.
It's *NOT* intended to be used for any normal/generic feature
or any optimisations that could be implemented across drivers.
Currently, only vendor commands (with replies) are supported,
no dump operations or vendor-specific notifications.
Also add a function wdev_to_ieee80211_vif() to mac80211 which
is needed for mac80211-based drivers wanting to implement any
vendor commands.

CRs-fixed: 576020
Change-Id: I5bdab7e0dce095f2d846687343f78cc0dd60aab1
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: ad7e718c9b4f717823fd920a0103f7b0fb06183f
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/linville/wireless-next.git


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