[3c90x] Stall upload engine before setting RX ring address

According to the 3c90x datasheet we have to stall the upload (receive)
engine before setting the receive ring address.

Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[3c90x] Don't round up transmit packet length

The 3c90x B and C revisions support rounding up the packet length to a
specific boundary.  Disable this feature to avoid overlength packets.

This fixes the loopback test.

Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[3c90x] Fix High-MTU packet reception

Prevent the card from flagging packets of 1518 bytes length as
overlength.

This fixes the High-MTU loopback test.

Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>