[realtek] Support RTL8139 cards within generic Realtek driver

[realtek] Support RTL8139 cards within generic Realtek driver

RTL8139C+ cards use essentially the same datapath as RTL8169, which is
zerocopy and 64-bit capable.  Older RTL8139 cards use a single receive
ring buffer rather than a descriptor ring, but still share substantial
amounts of functionality with RTL8169.

Include support for RTL8139 cards within the generic Realtek driver,
since there is no way to differentiate between RTL8139 and RTL8139C+
cards based on the PCI IDs alone.

Many thanks to all the people who worked on the rtl8139 driver over
the years.

Signed-off-by: Michael Brown <mcb30@ipxe.org>