crypto: mediatek - make mtk_sha_xmit() more generic

This is a transitional patch. It merges mtk_sha_xmit() and mtk_sha_xmit2()
to make transmit function more generic.
In addition, res->buf and cryp->tmp_dma in mtk_sha_xmit() are useless, since
crypto engine writes the result digests into ctx->tfm.digest instead of
res->buf. It's better to remove it.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>