clk: sunxi-ng: mux: Get closest parent rate possible with CLK_SET_RATE_PARENT

If the CLK_SET_RATE_PARENT flag is set, we want to try getting the
closest parent rate.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>