crypto: talitos - corrrectly handle zero-length assoc data

talitos does not handle well zero-length assoc data. From dmesg:
talitos ffe30000.crypto: master data transfer error
talitos ffe30000.crypto: gather return/length error

Check whether assoc data is provided by inspecting assoclen,
not assoc pointer.
This is needed in order to pass testmgr tests.

Signed-off-by: Horia Geanta <horia.geanta@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>