mailbox: mtk-cmdq: fix wrong use of sizeof in cmdq_get_clocks()

It should be size of the struct clk_bulk_data, not data pointer pass to
devm_kcalloc().

Fixes: aa1609f5 ("mailbox: mtk-cmdq: Dynamically allocate clk_bulk_data structure")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>