s390: remove unnecessary KERN_CONT

This has no effect as KERN_CONT is an empty string,

It's probably just a missing conversion artifact as the
other pr_cont uses in the same file don't have this prefix.

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