nouveau/gsp/r535: uninitialized variable in r535_gsp_acpi_mux_id()

The if we hit the "continue" statement on the first iteration through
the loop then "handle_mux" needs to be set to NULL so we continue
looping.

Fixes: 176fdcbd

 ("drm/nouveau/gsp/r535: add support for booting GSP-RM")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1d864f6e-43e9-43d8-9d90-30e76c9c843b@moroto.mountain