clk: mediatek: fix clk-gate flag setting

CLK_SET_RATE_PARENT would be dropped.
Merge two flag setting together to correct the error.

Fixes: 5a1cc4c2

 ("clk: mediatek: Add flags to mtk_gate")
Cc: <stable@vger.kernel.org>
Signed-off-by: Weiyi Lu <weiyi.lu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>