Sysclk handling in Rcc::CFGR::freeze() is inconsistent when a PLL is used

[Florian Hars]

As far as I understand the code, all clock calculations in freeze() use the last sysclk value specified with either the sysclk() or the sysclk_with_pll() methods, but if sysclk_wit_pll() has been called at least once, the actual sysclk used is the output_clock as calculated in let output_clock = vco / r;, which can be a completely different value