wifi: cfg80211: add flush functions for wiphy work

[ Upstream commit 56cfb8ce ]

There may be sometimes reasons to actually run the work
if it's pending, add flush functions for both regular and
delayed wiphy work that will do this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Stable-dep-of: eadfb547 ("wifi: mac80211: move sched-scan stop work to wiphy work")
Signed-off-by: Sasha Levin <sashal@kernel.org>