clk: sunxi-ng: mux: Change pre-divider application function prototype

The current function name is a bit confusing, and doesn't really allow to
create an explicit function to reverse the operation.

We also for now change the parent rate through a pointer, while we don't
return anything.

In order to be less confusing, and easier to use for downstream users,
change the function name to something hopefully clearer, and return the
adjusted rate instead of changing the pointer.

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