nl80211: Always register management frames handler

nl80211: Always register management frames handler

If registering WMM-AC ADDTS response action frame or WMM-AC DELTS
action frame fails, the management frame handler is not being
registered. This results with a segmentation fault when trying to
unsubscribe the handler when the interface is removed.
Fix it by always registering the handler and just returning a negative
value to note that the action frame could not be registered.

This fixes an issue introduced in the commit
dfa8787833f0b237f954b01233cad3527f3d4805 ('nl80211: Implement
add_ts/del_ts ops').

Signed-off-by: Avraham Stern <avraham.stern@intel.com>