[stm32l1] Don't use an intermediate clock in setup

This switch to HSI was an attempt to run the setup code faster. However,
there's no real need for this, and it just confuses things by switching first
to one clock, and then to another.  Just keep running on the existing clock
until switching to the clock actually chosen by the user's arguments.

lib/stm32/l1/rcc.c

@@ -515,9 +515,6 @@ void rcc_clock_setup_pll(const clock_scale_t *clock)
 	rcc_osc_on(HSI);
 	rcc_wait_for_osc_ready(HSI);
 
-	/* Select HSI as SYSCLK source. */
-	rcc_set_sysclk_source(RCC_CFGR_SW_SYSCLKSEL_HSICLK);
-
 	/*
 	 * Set prescalers for AHB, ADC, ABP1, ABP2.
 	 * Do this before touching the PLL (TODO: why?).