s390/cio: Make /dev/chsc a single-open device

In order to allow serialization of dynamic I/O with this patch
the /dev/chsc character device can only be accessed by one
single opener. Any subsequent open calls are rejected with EBUSY.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>