staging/rdma/hfi1: move txreq header code

The patch separates the txreq defines into new files, one for
verbs and one for sdma.

The verbs_txreq implementation handles the setup and teardown
of the txreq cache, so the register routine is changed to call
the new init/exit routines.

This patch allows for followup patches enhance the send engine.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>