crypto: ccree - remove struct buff_mgr_handle

The buff_mgr_handle structure contains only a single member, and only
one instance exists.  Simplify the code and reduce memory consumption by
moving this member to struct cc_drvdata.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>