RDMA/cxgb4: Only allow kernel db ringing for T4 devs

The whole db drop avoidance stuff is for T4 only.  So we cannot allow
that to be enabled for T5 devices.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>