crypto: marvell/cesa - easier way to get the transform

There's an easier way to get at the hash transform - rather than
using crypto_ahash_tfm(ahash), we can get it directly from
req->base.tfm.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>