IB/hfi1: Compute BTH only for RDMA_WRITE_LAST/SEND_LAST packet

In hfi1_rc_rcv(), BTH is computed for all packets received.
However, it's only used for packets received with opcodes
RDMA_WRITE_LAST and SEND_LAST, and it is a costly operation.

Compute BTH only in the RDMA_WRITE_LAST/SEND_LAST code path
and let the compiler handle endianness conversion for bitwise
operations.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>