usbnet: smsc95xx: fix link detection for disabled autonegotiation

To detect link status up/down for connections where autonegotiation is
explicitly disabled, we don't get an irq but need to poll the status
register for link up/down detection.
This patch adds a workqueue to poll for link status.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>