s390/topology: use for_each_set_bit to simplify the code

Use for_each_set_bit() to simplify the code.

spatch with a semantic match is used to found this.
(http://coccinelle.lip6.fr/

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>