r8169: simplify rtl8169_alloc_rx_data

dev->dev.parent has the same value as tp_to_dev(tp)
(set by SET_NETDEV_DEV() in rtl_init_one()) and we know it can't be NULL.
This allows us to simplify the code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>