s390/cio: get rid of variable length array

Use a flexible array instead. The size of the structure is not used
within chsc_sstpi, therefore no change in semantics but one less
sparse warning:

drivers/s390/cio/chsc.c:1219:27: warning: Variable length array is used.

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