stm32/rcc: remove builders on Config.

This makes API consistent with other Config structs in Embassy, where
the convention is to not use builders.

examples/stm32l0/src/bin/lorawan.rs

@@ -25,7 +25,7 @@ use lorawan_encoding::default_crypto::DefaultFactory as Crypto;
 
 fn config() -> embassy_stm32::Config {
     let mut config = embassy_stm32::Config::default();
-    config.rcc = config.rcc.clock_src(embassy_stm32::rcc::ClockSrc::HSI16);
+    config.rcc.mux = embassy_stm32::rcc::ClockSrc::HSI16;
     config
 }