RDMA/cxgb3: Wait at least one schedule cycle during device removal

During a hot-plug LLD removal event or an EEH error event, iw_cxgb3
must ensure that any/all threads that might be in a cxgb3 exported
function must return from the function before iw_cxgb3 returns from
its event processing.  Do this by calling synchronize_net().

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