crypto: brcm - Avoid double free in ahash_finup()

In Broadcom SPU driver, in case where incremental hash
is done in software in ahash_finup(), tmpbuf was freed
twice.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>