When chainloading rtl8139.pxe from an old Etherboot rtl8139.zrom, iPXE
can end up misreading the first word of the MAC address from the
EEPROM as being all zeroes.  This is presumably because Etherboot has
left the serial EEPROM in an unexpected state.

Fix by using the chip select line to reset the SPI device before we
start accessing it.

Reported-by: Mandar U Jog <mandarjog@gmail.com>
Tested-by: Mandar U Jog <mandarjog@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>