e1000: rename ICH8 flash macros

ICH8 will soon be followed by newer chipsets bearing the same acronym,
thus we remove the '8' and make it independent of the version number in
the platform name.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>