sfc: Correct address of gPXE boot configuration in EEPROM

Due to a hardware bug, the originally assigned range cannot reliably
be used for boot configuration and must not be modifiable through
ethtool.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>