IB/hfi1: Fix loss of BECN with AHG

AHG may be armed to use the stored header, which by design is limited
to edits in the PSN/A 32 bit word (bth2).

When the code is trying to send a BECN, the use of the stored header
will lose the BECN bit.

Fix by avoiding AHG when getting ready to send a BECN. This is
accomplished by always claiming the packet is not a middle packet which
is an AHG precursor.  BECNs are not a normal case and this should not
hurt AHG optimizations.

Cc: <stable@vger.kernel.org> # 4.14.x
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>