r8152: check RTL8152_UNPLUG

When the device is unplugged, the driver would try to disable the
device. Add checking the flag of RTL8152_UNPLUG to skip setting
the device when it is unplugged. This could shorten the time of
unloading the driver.

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