s390/sclp_sd: fix warnings about missing parameter description

Fix these warnings that are reported when compiling with W=1:

  drivers/s390/char/sclp_sd.c:132: warning: Function parameter or member 'listener' not described in 'sclp_sd_listener_init'
  drivers/s390/char/sclp_sd.c:408: warning: Function parameter or member 'cookie' not described in 'sclp_sd_file_update_async'
  drivers/s390/char/sclp_sd.c:422: warning: Function parameter or member 'attr' not described in 'reload_store'
  drivers/s390/char/sclp_sd.c:422: warning: Function parameter or member 'buf' not described in 'reload_store'
  drivers/s390/char/sclp_sd.c:422: warning: Function parameter or member 'count' not described in 'reload_store'
  drivers/s390/char/sclp_sd.c:457: warning: Function parameter or member 'file' not described in 'data_read'
  drivers/s390/char/sclp_sd.c:457: warning: Function parameter or member 'attr' not described in 'data_read'

Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>