IB/core: Fix SDR rates in sysfs

Commit 71eeba16 ("IB: Add new InfiniBand link speeds") introduced a bug
where eg the rate for IB 4X SDR links iss displayed as "8.5 Gb/sec"
instead of "10 Gb/sec" as it used to be.  Fix that.

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