x86/sev: Shorten GHCB terminate macro names

Shorten macro names for improved readability.

Suggested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Venu Busireddy <venu.busireddy@oracle.com>
Link: https://lkml.kernel.org/r/20211110220731.2396491-5-brijesh.singh@amd.com