[S390] tape class return value handling.

Without this patch register_tape_dev() will always fail, but might
return a value that is not an error number. This will lead to accesses
to already freed memory areas...

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