phylink: fix locking asserts

Use ASSERT_RTNL() rather than WARN_ON(!lockdep_rtnl_is_held()) which
stops working when lockdep fires, and we end up with lots of warnings.

Fixes: 9525ae83 ("phylink: add phylink infrastructure")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>