IB/hfi1: Initialize bth1 in 16B rc ack builder

It is possible the bth1 variable could be used uninitialized so going
ahead and giving it a default value.

Otherwise we leak stack memory to the network.

Fixes: 5b6cabb0 ("IB/hfi1: Add 16B RC/UC support")
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>