r8152: set RTL8152_UNPLUG only for real disconnection

Set the flag of RTL8152_UNPLUG if and only if the device is unplugged.
Some error codes sometimes don't mean the real disconnection of usb device.
For those situations, set the flag of RTL8152_UNPLUG causes the driver skips
some flows of disabling the device, and it let the device stay at incorrect
state.

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