clk: mediatek: clk-mux: Do not pass flags to clk_mux_determine_rate_flags()

The `flags` in |struct mtk_mux| are core clk flags, not mux clk flags.
Passing one to the other is wrong.

Since there aren't any actual users adding CLK_MUX_* flags, just drop it
for now.

Fixes: b05ea331 ("clk: mediatek: clk-mux: Add .determine_rate() callback")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>