IB: Use central enum for speed instead of hard-coded values

The kernel IB stack uses one enumeration for IB speed, which wasn't
explicitly specified in the verbs header file.  Add that enum, and use
it all over the code.

The IB speed/width notation is also used by iWARP and IBoE HW drivers,
which use the convention of rate = speed * width to advertise their
port link rate.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>