target-openrisc: Clean up triple QOM casts

Instead of calling openrisc_env_get_cpu(), casting to CPU() via the
ENV_GET_CPU() compatibility macro and casting back to OPENRISC_CPU(),
just call openrisc_env_get_cpu() directly.

ENV_GET_CPU() is meant as workaround for target-independent code only.

Signed-off-by: Andreas Färber <afaerber@suse.de>