s390/cpumf: simplify psw generation

Use the psw_bits macro and simplify the code. The generated code is
also better since it doesn't contain any conditional branches anymore.

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