crypto: atmel-sha - fix crash when computing digest on empty message

This patch fixes a crash which occured during the computation of the
digest of an empty message.

Indeed, when processing an empty message, the atmel_sha_handle_queue()
function was never called, hence the dd->req pointer remained
uninitialized.

Later, when the atmel_sha_final_req() function was called, it used
to crash while using this uninitialized dd->req pointer.

Hence this patch adds missing initializations of dd->req before calls of
the atmel_sha_final_req() function.

This bug prevented us from passing the tcrypt test suite on SHA algo.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>