[S390] hvc_iucv: Update and add missing kernel messages

If the hvc_iucv= kernel parameter specifies a value that is not
valid, display an error message.
Minor changes to existing kernel messages.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>