[S390] cio: subchannel evaluation function operates without lock

css_evaluate_subchannel() operates subchannel without lock which can
lead to erratic behavior caused by concurrent device access. Also
split evaluation function to make it more readable.

Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>