s390/crypto: remove 'const' to avoid compiler warnings

In aes_encrypt() and aes_decrypt(), need let 'sctx->key' be modified,
so remove 'const' for it. The related warnings:

    CC [M]  arch/s390/crypto/aes_s390.o
  arch/s390/crypto/aes_s390.c: In function 'aes_encrypt':
  arch/s390/crypto/aes_s390.c:146:37: warning: passing argument 2 of 'crypt_s390_km' discards 'const' qualifier from pointer target type [-Wdiscarded-array-qualifiers]
     crypt_s390_km(KM_AES_128_ENCRYPT, &sctx->key, out, in,
                                       ^
  ...

  In file included from arch/s390/crypto/aes_s390.c:29:0:
  arch/s390/crypto/crypt_s390.h:154:19: note: expected 'void *' but argument is of type 'const u8 (*)[32] {aka const unsigned char (*)[32]}'
   static inline int crypt_s390_km(long func, void *param,
                     ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>