drivers/clock_control: stm32u5: Fix VCO setting

When existing stop mode 1&2, VCO is set to range 4
and should be set back to range 1 to allow full speed
operations.
Rather than setting VCO at startup, set it inside clock
setting procedure so that it could done
in clock reset procedure when existing stop modes.

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