usbnet: smsc95xx: fix broken runtime suspend

Commit b2d4b150(smsc95xx: enable dynamic autosuspend) implements
autosuspend, but breaks current runtime suspend, such as:
when the interface becomes down, the usb device can't be put into
runtime suspend any more.

This patch fixes the broken runtime suspend.

Cc: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>