mailbox: fix gce_num of mt8192 driver data

Because mt8192 only have 1 gce, the gce_num should be 1.

Fixes: 85dfdbfc ("mailbox: cmdq: add multi-gce clocks support for mt8195")
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>