clk: mediatek: pllfh: Fix IO remapping leak in register_pllfhs error path

In the Linux kernel, the following vulnerability has been resolved:

clk: mediatek: pllfh: Fix IO remapping leak in register_pllfhs error path

When mtk_clk_register_pllfhs function fails to register a PLL, it
unregisters all PLLs and cleans up itself in its error path before
returning, so the function callers don't need to do it.

But contrary to mtk_clk_unregister_pllfhs function, that does almost
the same sequence, it does not free the IO memory mapped on fhctl node,
leading to a leak.

Fix this leak by factorizing the cleanup sequence in a new private
function and use it both mtk_clk_register_pllfhs and
mtk_clk_unregister_pllfhs functions.

Also, change the loop index start value to avoid the -1 operation on
index at each loop.