mailbox: mtk-gpueb: Add missing 'static' to mailbox ops struct

mtk_gpueb_mbox_ops should be declared static. However, due to its const
nature, this specifier was missed, as it compiled fine without it and
with no warning by the compiler.

arc-linux-gcc (GCC) 12.5.0 doesn't seem to like it however, so add the
static to fix that.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510100629.3nGvrhEU-lkp@intel.com/
Fixes: dbca0ea ("mailbox: add MediaTek GPUEB IPI mailbox")
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: Nicolas Frattaroli

drivers/mailbox/mtk-gpueb-mailbox.c

@@ -200,7 +200,7 @@ static bool mtk_gpueb_mbox_last_tx_done(struct mbox_chan *chan)
 	return !(readl(ch->ebm->mbox_ctl + GPUEB_MBOX_CTL_TX_STS) & BIT(ch->num));
 }
 
-const struct mbox_chan_ops mtk_gpueb_mbox_ops = {
+static const struct mbox_chan_ops mtk_gpueb_mbox_ops = {
 	.send_data = mtk_gpueb_mbox_send_data,
 	.startup = mtk_gpueb_mbox_startup,
 	.shutdown = mtk_gpueb_mbox_shutdown,