setup: correct suffix for sed

Change backup of grub settings from /etc/default/grubbak to
/etc/default/grub.bak.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Suggested-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>

Change-Id: Ib5f1f2e838d459b9767409f42fc0b6c7a05a5515