[S390] cio: Use locks when accessing /sys/firmware/cpi data.

Mutex locks are used to avoid problems when /sys/firmware/cpi data
are accessed to in parallel.

Signed-off-by: Michael Ernst <mernst@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>