cxgb4: Fixes cxgb4_inet6addr_notifier unregister call

commit b5a02f50 ("cxgb4 : Update ipv6 address handling api") introduced
a regression where unregister cxgb4_inet6addr_notifier wasn't getting called
during module_exit.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>