s390/compat: fix setup_frame32

Git commit c60a03fe ("s390: switch to {get,put}_compat_sigset()")
contains a typo and now copies the wrong pointer to user space.
Use the correct pointer instead.

Reported-and-tested-by: Stefan Liebler <stli@linux.vnet.ibm.com>
Fixes: c60a03fe ("s390: switch to {get,put}_compat_sigset()")
Cc: <stable@vger.kernel.org> # v4.15+
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>