[S390] monwriter: fix return code handling

Fix return code handling within monwrite_new_hdr(). Return code handling
is everwhere implemented, the return code of the diagnose function was
just not passed.

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