clk: sunxi-ng: nkm: Prefer current parent rate

Similar to ccu_mp, if the current parent rate allows getting the ideal
rate, prefer to not change the parent clock's rate.

Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
Link: https://lore.kernel.org/r/20230807-pll-mipi_set_rate_parent-v6-11-f173239a4b59@oltmanns.dev
Signed-off-by: Chen-Yu Tsai <wens@csie.org>