iwl4965: fix "Received BA when not expected"

Need to use broadcast sta_id for management frames, otherwise we broke
BA session in the firmware and get messages like that:

"Received BA when not expected"

or (on older kernels):

"BA scd_flow 0 does not match txq_id 10"

This fix regression introduced in 2.6.35 during station management
code rewrite by:

commit 2a87c26b


Author: Johannes Berg <johannes.berg@intel.com>
Date:   Fri Apr 30 11:30:45 2010 -0700

    iwlwifi: use iwl_find_station less

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>