RDMA/cxgb3: Fix regression caused by class_device -> device conversion

The change to iwch_provider.c in commit f4e91eb4 ("IB: convert struct
class_device to struct device") undid the fix done in commit 7f049f2f


("RDMA/cxgb3: Hold rtnl_lock() around ethtool get_drvinfo call").  It
removed the calls to rtnl_lock() that serialized the iw_cxgb3 ethtool
ops calls into the cxgb3 driver.  This locking is needed to avoid
messing up the internal state of the cxgb3 driver.

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