iwlwifi: do not clear TX info flags when receiving BlockAckResponse

OR-in AMPDU flags rather than assigning them.  This lets the TX status for
aggregated packets be processed by rs_tx_status.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>