RDMA/cxgb3: Hold rtnl_lock() around ethtool get_drvinfo call

Currently the call into cxgb3 to get the driver info is not serialized.
The iw_cxgb3 module needs to hold the rtnl_lock around the ethtool ops
call like dev_ioctl() does.

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