[SCSI] aic94xx: handle REQ_TASK_ABORT

This patch straightens out the code that distinguishes the various escb
opcodes in escb_tasklet_complete so that they can be handled correctly.
It also provides all the necessary code to create a workqueue item that
tells libsas to abort a sas_task.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>