sfc: Work around unreliable strap pins

The SFC4000 has strap pins indicating the presence of SPI flash and/or
EEPROM.  These pins are also used for GPIO, and in some cases they may
be read wrongly at reset.  However, on production boards it must boot
from one or the other device, so we can assume the boot device is
present and read the board config from there.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>