crypto: img-hash - Fix variable dereferenced before check 'hdev->req'

Smatch report warning as follows:

drivers/crypto/img-hash.c:366 img_hash_dma_task() warn: variable
dereferenced before check 'hdev->req'

Variable dereferenced should be done after check 'hdev->req',
fix it.

Fixes: d358f1ab ("crypto: img-hash - Add Imagination Technologies hw hash accelerator")
Fixes: 10badea2 ("crypto: img-hash - Fix null pointer exception")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>