crypto/armv8: fix clang build

crypto/armv8: fix clang build

1. Clang requires braces around initialization of subobject.
2. Clang complains implicit conversion of enumeration type.

Trapped issue with Clang version 8.0 and CONFIG_RTE_LIBRTE_PMD_ARMV8_CRYPTO
was set.
Error messages:
rte_armv8_pmd.c:144:2: error: suggest braces around initialization of
 subobject [-Werror,-Wmissing-braces]
        NULL
        ^~~~
        {   }
/usr/lib/llvm-8/lib/clang/8.0.0/include/stddef.h:105:16: note: expanded
 from macro 'NULL'
               ^~~~~~~~~~
rte_armv8_pmd.c:429:21: error: implicit conversion from enumeration
 type 'enum rte_crypto_cipher_operation' to different enumeration type
      'enum armv8_crypto_cipher_operation' [-Werror,-Wenum-conversion]
        cop = sess->cipher.direction;
            ~ ~~~~~~~~~~~~~^~~~~~~~~

Fixes: 169ca3db550c ("crypto/armv8: add PMD optimized for ARMv8 processors")
Cc: stable@dpdk.org
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>