clk: sunxi-ng: mp: Adjust parent rate for pre-dividers

The MP style clocks support an mux with pre-dividers. While the driver
correctly accounted for them in the .determine_rate callback, it did
not in the .recalc_rate and .set_rate callbacks.

This means when calculating the factors in the .set_rate callback, they
would be off by a factor of the active pre-divider. Same goes for
reading back the clock rate after it is set.

Cc: stable@vger.kernel.org
Fixes: 2ab836db ("clk: sunxi-ng: Add M-P factor clock support")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>