mac80211: fix TCP performance on mesh interface

sta is NULL for mesh point (resolved later), so sk pacing parameters
were not applied.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Link: https://lore.kernel.org/r/66f51659416ac35d6b11a313bd3ffe8b8a43dd55.camel@freebox.fr


Signed-off-by: Johannes Berg <johannes.berg@intel.com>