cxgb4: remove bond->lock

RTNL should be already held in the notifier call so the slave list can
be traversed without a problem, remove the unnecessary bond->lock.

CC: Hariprasad S <hariprasad@chelsio.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>