s390/nmi: reduce size of percpu variable

Change the flag fields within struct mcck_struct to simple bit fields
to reduce the size of the structure which is used as percpu variable.

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