The current driver allows user to queue up multiple segments
on to a single transaction descriptor. User will submit this single desc
and in the issue_pending() we decode multiple segments and submit to SG HW engine.
We free up the allocated_desc when it is submitted to the HW.
Existing code prevents the user to prepare multiple trasactions at same time as
we are overwrite with the allocated_desc.
The best utilization of HW SG engine would happen if we collate the pending
list when we start dma this patch updates the same.