IB/core: Don't return EINVAL from sysfs rate attribute for invalid speeds

Commit e9319b0c ("IB/core: Fix SDR rates in sysfs") changed our
sysfs rate attribute to return EINVAL to userspace if the underlying
device driver returns an invalid rate.  Apparently some drivers do this
when the link is down and some userspace pukes if it gets an error when
reading this attribute, so avoid a regression by not return an error to
match the old code.

Signed-off-by: Roland Dreier <roland@purestorage.com>