r8152: fix setting RTL8152_UNPLUG

The flag of RTL8152_UNPLUG should only be set when the device is
unplugged, not each time the rtl8152_disconnect() is called.
Otherwise, the device wouldn't be stopped normally.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>