crypto: atmel - only treat EBUSY as transient if backlog

The Atmel SHA driver was treating -EBUSY as indication of queueing
to backlog without checking that backlog is enabled for the request.

Fix it by checking request flags.

Cc: <stable@vger.kernel.org>
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>