wifi: cfg80211: fix kernel-doc for wiphy_delayed_work_flush()

Clearly, there's no space in the function name, not sure how
that could've happened. Put the underscore that it should be.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 56cfb8ce ("wifi: cfg80211: add flush functions for wiphy work")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>