On devices with no EEPROM or OTP, the MAC_CR register defaults to not
using automatic link speed detection, with the result that no packets
are successfully sent or received.

Fix by always enabling automatic speed and duplex detection, since
iPXE provides no mechanism for manual configuration of either link
speed or duplex.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Originally-implemented-by: Ravi Hegde <ravi.hegde@microchip.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>