crypto: caam/qi2 - fix zero-length buffer DMA mapping

Commit 04e6d25c ("crypto: caam - fix zero-length buffer DMA mapping")
fixed an issue in caam/jr driver where ahash implementation was
DMA mapping a zero-length buffer.

Current commit applies a similar fix for caam/qi2 driver.

Cc: <stable@vger.kernel.org> # v4.20+
Fixes: 3f16f6c9 ("crypto: caam/qi2 - add support for ahash algorithms")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>