crypto: omap-sham - uses digest buffer in request context

Currently driver storred digest results in req->results
provided by the client. But some clients do not set it
until final() call. It leads to crash.
Changed to use internal buffer to store temporary digest results.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>