RDMA/cxgb4: Set/reset the EP timer inside EP lock

Endpoint timer manipulation needs to be done inside the lock.  Otherwise
we can get into a situation where a timer is stopped before it is started,
which hits the WARN_ON() in stop_ep_timer().

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