rtl8139: do not assume TxStatus[] and TxAddr[] are adjacent

Commit afe0a595356192d5f79703cf6462fcc112df007c ("rtl8139: support byte
read to TxStatus registers") reused rtl8139_TxStatus_read() for reading
TxAddr registers.  It relies on the fact that TxStatus[] and TxAddr[]
are adjacent.

This causes a gcc warning because the compiler can detect that array
access is out-of-bounds:

  hw/rtl8139.c:2501:27: error: array subscript is above array bounds [-Werror=array-bounds]

This patch refactors the function so that we don't rely on out-of-bounds
accesses.

Cc: Jason Wang <jasonwang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
