crypto: mxs-dcp - make symbols 'sha1_null_hash' and 'sha256_null_hash' static

Fixes the following sparse warnings:

drivers/crypto/mxs-dcp.c:39:15: warning:
 symbol 'sha1_null_hash' was not declared. Should it be static?
drivers/crypto/mxs-dcp.c:43:15: warning:
 symbol 'sha256_null_hash' was not declared. Should it be static?

Fixes: c709eeba

 ("crypto: mxs-dcp - Fix SHA null hashes and output length")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>