[SCSI] mpt2sas : Broadcast Primative AEN bug fix

Bug fix in the broadcast primative async event code where the driver would
stop sending tm queries after the first queury was completed. This was due
driver not reseting the tm_cmds.status field back to MPT2_CMD_NOT_USED after
completing a task management request.

An addtional fix adding sanity check to insure sas_device->starget set to NULL.
During multipath testing fail over/fail back, the mid layer was holding onto
sdev longer than the fail back period, thus starget was getting set to NULL
for device being added.

Signed-off-by: Eric Moore <eric.moore@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>