crypto: des_s390 - Checkpatch cleanup

arch/s390/crypto/crypto_des.h:18: ERROR: do not use C99 // comments
Signed-off-by: NAndrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

arch/s390/crypto/crypto_des.h

@@ -15,4 +15,4 @@
 
 extern int crypto_des_check_key(const u8*, unsigned int, u32*);
 
-#endif //__CRYPTO_DES_H__
+#endif /*__CRYPTO_DES_H__*/