drivers: stm32 clock control: fix HSI PLLSOURCE with PREDIV1 support

The combination of
CONFIG_CLOCK_STM32_SYSCLK_SRC_PLL and
CONFIG_CLOCK_STM32_PLL_SRC_HSI
on SOCs with PREDIV1 support made use of the LL define
LL_RCC_PLLSOURCE_HSI_DIV_2, which is not defined for SOCs with
PREDIV1 support.

This exchanges LL_RCC_PLLSOURCE_HSI_DIV_2 with LL_RCC_PLLSOURCE_HSI
which is the appropiate source according to stm32f0xx_ll_rcc.h
line 473 and stm32f3xx_ll_rcc.h line 795.

Tested by compiling hello world for nucleo_f091rc board with HSI as
PLLSOURCE.

Signed-off-by: Daniel Wagenknecht <wagenknecht@clage.de>