staging/rdma/hfi1: Don't attempt to qualify or tune loopback plugs

Loopback plugs used for testing hardware don't need to be qualified to
bring the link up unlike production cables. This patch adds an exception
for loopback plugs to the QSFP and SerDes tuning algortihm.

Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Easwar Hariharan <easwar.hariharan@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>