iwmc3200wifi: 802.11n Tx aggregation support

To support 802.11n Tx aggregation support with iwmc3200 wifi, we have to
handle the UMAC_CMD_OPCODE_STOP_RESUME_STA_TX notification from the UMAC.
Before sending an AddBA, the UMAC synchronizes with the host in order to
know what is the last Tx frame it's supposed to receive before it will be
able to start the actual aggregation session.
We thus have to keep track of the last sequence number that is scheduled
for transmission on a particular RAxTID, send an answer to the UMAC with
this sequence number. The UMAC then does the BA negociation and once it's
done with it sends a new UMAC_CMD_OPCODE_STOP_RESUME_STA_TX notification
to let us know that we can resume the Tx flow on the specified RAxTID.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Reviewed-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>