mac8211: fix struct initialisation

Sparse showed up with the following error.
net/mac80211/agg-rx.c:480:43: warning: Using plain integer as NULL pointer

Fixes: 2ab45876 (mac80211: add support for the ADDBA extension element)
Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20200803084540.179908-1-john@phrozen.org


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