[S390] Fix sclp_vt220 error handling.

Also convert to slab_is_available() as an indicator if
get_zeroed_page() will work or not.

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