i945: Use defines for DEVEN

Change-Id: I32461449354155510c0e14e9d0ce396068ea50d4
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5890
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Idwer Vollering <vidwer@gmail.com>