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: NAndreas Färber <afaerber@suse.de>