RDMA/cxgb3: Make the iw_cxgb3 module parameters writable

Allow changing parameter values without having to reload the module.
This is safe because these parameters are only looked at when a new
connection is established.

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