IB/hfi1: Ensure ucast_dlid access doesnt exceed bounds

The dlid assignment made by looking into the u_ucast_dlid array does not
do an explicit check for the size of the array. The code path to arrive at
def_port, the index value is long and complicated so its best to just have
an explicit check here.

Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>