drivers: clock_control: stm32u5: keep reset values of MSI trimming

When MSI trimming values where set to 0,
and MSIS is used as system core clock and MSI > 4 MHz,
it causes uart to fail.
There is no need to set thoses trimming values.
So keep the default reset value. (keep ST Factory calibration)

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>