RDMA/core: Add encode/decode FDR/EDR rates

The cases for FDR/EDR signalling speed, were missing in
ib_rate_to_mult and mult_to_ib_rate giving wrong return values when
drivers convert static rate to/from inter-packet-delay.

Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>