hw/eepro100.c: Use extended TBD only where applicable

Bug fix for segfault when run as i82551 HW:
Use Extended TBD only when HW supports it (i82558 and up).

Added assertions to guard from such buffer overflow
Introduce the MAX_TCB_BYTE_COUNT macro
Allocate buf big enough as HW needs (MAX_ETH_FRAME_SIZE -> MAX_TCB_BYTE_COUNT)

I don't feel 100% OK with the "s->device >= i82558B" condition
since it relies on the numeric (hex) value of those defines, which currently
is correct, but changes (which I don't forsee now) might break it.

Signed-off-by: Naphtali Sprei <nsprei@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
