smsc75xx: mark link down on startup and let PHY interrupt deal with carrier changes

This patch fixes the same issue as reported on smsc95xx, where the
usb device is connected with no ethernet cable plugged-in.

Without this patch sysfs reports the cable as present

flag@flag-desktop:~$ cat /sys/class/net/eth0/carrier
1

while it's not:

flag@flag-desktop:~$ sudo mii-tool eth0
eth0: no link

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>