BACKPORT: mac80211: add option for setting control flags

Allows setting of control flags of skb cb - if needed -
when calling ieee80211_subif_start_xmit().

Tested-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

(cherry picked from commit 06016772)

BUG=b:134764458
TEST=manually test and verified by sniffer on mistral
Change-Id: Id86318206bc8ee34ebd8ee619d5eb8b82c0bf54e
Signed-off-by: Roy Luo <royluo@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1650689


Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Julan Hsu <julanhsu@google.com>