[intel] Use autoloaded MAC address instead of EEPROM MAC address

The i350 (and possibly other Intel NICs) have a non-trivial
correspondence between the PCI function number and the external
physical port number.  For example, the i350 has a "LAN Function Sel"
bit within the EEPROM which can invert the mapping so that function 0
becomes port 3, function 1 becomes port 2, etc.

Unfortunately the MAC addresses within the EEPROM are indexed by
physical port number rather than PCI function number.  The end result
is that when anything other than the default mapping is used, iPXE
will use the wrong address as the base MAC address.

Fix by using the autoloaded MAC address if it is valid, and falling
back to reading the MAC address directly from the EEPROM only if no
autoloaded address is available.

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