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>

drivers/s390/cio/chsc.c

@@ -1216,7 +1216,7 @@ int chsc_sstpi(void *page, void *result, size_t size)
 		struct chsc_header request;
 		unsigned int rsvd0[3];
 		struct chsc_header response;
-		char data[size];
+		char data[];
 	} __attribute__ ((packed)) *rr;
 	int rc;