r8152: reset tp->speed before autoresuming in open function

If (tp->speed & LINK_STATUS) is not zero, the rtl8152_resume()
would call rtl_start_rx() before enabling the tx/rx. Avoid this
by resetting it to zero.

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