rcc_clock_setup_pll with HSE: enable HSE. 

BUG fix: rcc_clock_setup_pll with clock source HSE: enable HSE oscillator before switching clock source to PLL.

If not done, calling this function with clock set to HSE => CPU seems to stop running.


E.g., with an 8 MHz external crystal:
	 clock_scale_t config_hse_no_usb_32MHz = {
				.pll_source = RCC_CFGR_PLLSRC_HSE_CLK,
				.pll_mul = RCC_CFGR_PLLMUL_MUL8,
				.pll_div = RCC_CFGR_PLLDIV_DIV2,
				.hpre = RCC_CFGR_HPRE_SYSCLK_NODIV,
				.ppre1 = RCC_CFGR_PPRE1_HCLK_NODIV,
				.ppre2 = RCC_CFGR_PPRE2_HCLK_NODIV,
				.voltage_scale = RANGE1,
				.flash_config = FLASH_ACR_LATENCY_1WS,
				.apb1_frequency = 32000000,
				.apb2_frequency = 32000000,
	 };


	rcc_clock_setup_pll(&config_hse_no_usb_32MHz); 

Remark: USB clock source is not 48 MHz in this configuration but at 64 MHz. USB periph. requires a clock at 48 MHz