s390/nmi: keep comments consistent

The name of bit 36 of the machine check interruption code is "guarded
storage registers validity". Add the missing "validity" part in order
to be consistent with all other comments, which include this piece of
information.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>