IB: simplify static rate encoding

Push translation of static rate to HCA format into low-level drivers,
where it belongs.  For static rate encoding, use encoding of rate
field from IB standard PathRecord, with addition of value 0, for
backwards compatibility with current usage.  The changes are:

 - Add enum ib_rate to midlayer includes.
 - Get rid of static rate translation in IPoIB; just use static rate
   directly from Path and MulticastGroup records.
 - Update mthca driver to translate absolute static rate into the
   format used by hardware.  This also fixes mthca's static rate
   handling for HCAs that are capable of 4X DDR.

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>