[S390] cio: Get rid of css_characteristics_avail.

Checking for the validity of the css_*_characteristics is
superfluous since they stay 0 for non-successful scsc.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>