- use pci_alloc_consistent() for Rx/Tx descriptors in rtl8169_open()
  (balanced by pci_free_consistent() on error path as well as in
  rtl8169_close());
- removal of the fields {Rx/Tx}DescArrays in struct rtl8169_private
  as there is no need to store a non-256 bytes aligned address any more;
- fix for rtl8169_open() leak when RxBufferRings allocation fails.
  Said allocation is pushed to rtl8169_init_ring() as part of an evil
  cunning plan.