drivers: clock_control: stm32: Set flash latency before increasing clocks

Latency should be set before HCLK clock increase. Not doing so can result
in broken behavior.
For instance, at startup, MSI is @4MHz on L4 series.
If MSI is required to be configured at 48 MHz for future use a USB clock,
this will be done in set_up_fixed_clock_sources(). If flash latency is
not correctly set at this point fetching flash will fail..

Move flash latency configuration before setting up fixed clocks.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>