s390/zcrypt: Fix warning about wrong kernel doc comment

s390/zcrypt: Fix warning about wrong kernel doc comment

Fix this warning:
  Warning: drivers/s390/crypto/zcrypt_msgtype6.c:1253 This comment
  starts with '/**', but isn't a kernel-doc comment. Refer to
  Documentation/doc-guide/kernel-doc.rst

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603252022.vEojGo3V-lkp@intel.com/
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>