r8152: fix the list rx_done may be used without initialization

The list rx_done would be initialized when the linking on occurs.
Therefore, if a napi is scheduled without any linking on before,
the following kernel panic would happen.

	BUG: unable to handle kernel NULL pointer dereference at 000000000000008
	IP: [<ffffffffc085efde>] r8152_poll+0xe1e/0x1210 [r8152]
	PGD 0
	Oops: 0002 [#1] SMP

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