drivers: clock_control: stm32u5: Switch to HSI only when needed

When configuring PLL, we should first make sure we're not running on PLL,
and if running on PLL, first switch to a fixed clock before proceeding
with PLL configuration.
Current code is doing the switch systematically which is not useful as
default startup case is to use MSI as sysclk source.
So add a test before doing this switch.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>