IB/hfi1: Add software counter for ctxt0 seq drop

IB/hfi1: Add software counter for ctxt0 seq drop

All other code paths increment some form of drop counter.

This was missed in the original implementation.

Fixes: bfcaa3e7ea10 ("staging/rdma/hfi1: Handle packets with invalid RHF on context 0")
Link: https://lore.kernel.org/r/20200106134228.119356.96828.stgit@awfm-01.aw.intel.com
Reviewed-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>