IB/hfi1: Cleanup tasklet refs in comments

The code no longer uses tasklets for the send engine. However it does
use a tasklet for sdma but the send routines use a workqueue now days.
Update the comments to reflect that. Make things more generic with
saying "send engine" because that is what is being referred to.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>