smsc95xx: fix stalled rx after link change

After commit 05b35e7e ("smsc95xx: add phylib support"), link changes
are no longer propagated to usbnet. As a result, rx URB allocation won't
happen until there is a packet sent out first (this might never happen,
e.g. running just ssh server with a static IP). Fix by triggering usbnet
EVENT_LINK_CHANGE.

Fixes: 05b35e7e ("smsc95xx: add phylib support")
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>