[realtek] Use ID word to detect EEPROM presence

Some onboard RTL8169 NICs seem to leave the EEPROM pins disconnected.
The existing is_valid_ether_addr() test will not necessarily catch
this, since it expects a missing EEPROM to show up as a MAC address of
00:00:00:00:00:00 or ff:ff:ff:ff:ff:ff.  When the EEPROM pins are
floating the MAC address may read as e.g. 00:00:00:00:0f:00, which
will not be detected as invalid.

Check the ID word in the first two bytes of the EEPROM (which should
have the value 0x8129 for all RTL8139 and RTL8169 chips), and use this
to determine whether or not an EEPROM is present.

Reported-by: Carl Karsten <carl@nextdayvideo.com>
Tested-by: Carl Karsten <carl@nextdayvideo.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>