IB/mad: Fix RMPP version check during agent registration

Only check that RMPP version is not specified when MAD class does not
support RMPP.  Just because a class is allowed to use RMPP doesn't
mean that rmpp_version needs to be set for the MAD agent to
register. Checking this was a recent change which was too pedantic.

Signed-off-by: Hal Rosenstock <halr@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>