net: rtl8139: Introduce device private data

Introduce rtl8139_pdata, which is a super-structure around eth_device
and tracks per-device state, here the device IO address, PCI BDF, RX
and TX ring position. Pass this structure around instead of the old
non-DM eth_device in preparation for DM conversion.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>