s390: fix 'novx' early parameter handling

Specifying the 'novx' kernel parameter always results in a warning:

Malformed early option 'novx'

The reason for this is that the novx early parameter handling function
always returns a non-zero value which means that an error occurred.
Fix this and return the correct zero value instead.

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