crypto: marvell/cesa - Avoid empty transfer descriptor

[ Upstream commit 1bafd82d ]

The user may set req->src even if req->nbytes == 0.  If there
is no data to hash from req->src, do not generate an empty TDMA
descriptor.

Fixes: db509a45 ("crypto: marvell/cesa - add TDMA support")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>