s390/compat: fix PSW32_USER_BITS definition

PSW32_USER_BITS should define the primary address space for user space
instead of the home address space.
Symptom of this bug is that gdb doesn't work in compat mode.

The bug was introduced with e258d719 "s390/uaccess: always run the kernel
in home space" and f26946d7

 "s390/compat: make psw32_user_bits a constant
value again".

Cc: stable@vger.kernel.org # v3.13+
Reported-by: Andreas Arnez <arnez@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>