crypto: picoxcell - fix possible invalid pointer dereference

The completion callback will free the request so we must remove it from
the completion list before calling the callback.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>