r8169: don't take RTNL lock in rtl_task()

There's not really a benefit here in taking the RTNL lock. The task
handler does exception handling only, so we're in trouble anyway when
we come here, and there's no need to protect against e.g. a parallel
ethtool call.
A benefit of removing the RTNL lock here is that we now can
synchronously cancel the workqueue from a context holding the RTNL mutex.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>