crypto: caam - fix error path for ctx_dma mapping failure

In case ctx_dma dma mapping fails, ahash_unmap_ctx() tries to
dma unmap an invalid address:
map_seq_out_ptr_ctx() / ctx_map_to_sec4_sg() -> goto unmap_ctx ->
-> ahash_unmap_ctx() -> dma unmap ctx_dma

There is also possible to reach ahash_unmap_ctx() with ctx_dma
uninitialzed or to try to unmap the same address twice.

Fix these by setting ctx_dma = 0 where needed:
-initialize ctx_dma in ahash_init()
-clear ctx_dma in case of mapping error (instead of holding
the error code returned by the dma map function)
-clear ctx_dma after each unmapping

Fixes: 32686d34

 ("crypto: caam - ensure that we clean up after an error")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>