Inverting the write ordering of the TxDescAddr{High/Low} registers
suffices to trigger a sabbat of PCI errors which make the device
completely dysfunctional. The issue has not been reported on a
different platform.

Switching from MMIO accesses to I/O ones as done in Realtek's
own driver fixes (papers over ?) the bug as well but I am not
thrilled to see everyone pay the I/O price for an obscure bug.

This is the minimal change to handle the issue.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Reported-by: Lennert Buytenhek <buytenh@wantstofly.org>