clk: sunxi: Fix M factor computation for APB1

commit cfa63688 ("clk: sunxi: factors: Consolidate get_factors
parameters into a struct") introduced a regression for m factor
computation in sun4i_get_apb1_factors function.

The old code reassigned the "parent_rate" parameter to the targeted
divisor value and was buggy for the returned frequency but not for the
computed factors. Now, returned frequency is good but m factor is
incorrectly computed (its max value 31 is always set resulting in a
significantly slower frequency than the requested one...)

This patch simply restores the original proper computation for m while
keeping the good changes for returned rate.

Fixes: cfa63688 ("clk: sunxi: factors: Consolidate get_factors parameters into a struct")
Signed-off-by: Stéphan Rafin <stephan@soliotek.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>