cfg80211: add various struct element finding helpers

We currently have a number of helpers to find elements that just
return a u8 *, change those to return a struct element and add
inlines to deal with the u8 * compatibility.

Note that the match behaviour is changed to start the natch at
the data, so conversion from _ie_match to _elem_match need to
be done carefully.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Change-Id: I3b63fe261116dd823cb5707b752f86e30710065c
CRs-fixed: 2399972
Git-commit: 49a68e0d88890060a2b9b6c6ad1ec53eb50abccf
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


Signed-off-by: Peng Xu <pxu@codeaurora.org>