s390/diag: fix diagnose 8c description

The comment above diag8c() describes diagnose 210, not diagnose 8c.
Add a proper short description.

Reviewed-by: Sven Schnelle <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/kernel/diag.c

···
 EXPORT_SYMBOL(diag210);
 
 /*
- * Diagnose 210: Get information about a virtual device
+ * Diagnose 8C: Access 3270 Display Device Information
  */
 int diag8c(struct diag8c *addr, struct ccw_dev_id *devno)
 {