PLL settings remained intact because these bits are not writable when PLL is enabled,
but prescaler settings were overwritten by selecting PLL as sysclk (CFGR.SW[1:0]).
The rcc code was taken from stm32-rs which uses 'x' features, but
embassy uses features with full chip names.
Add these 'x' wildcards as cfgs and use them in rcc.
They will be useful for USB too.
