drivers/clock_control: stm32: style edits on set_up_fixed_clock_sources()

Review code style in set_up_fixed_clock_sources() for better
readability.
Use of 'if (IS_ENABLED(STM32_MSI_ENABLED))' inside '#if STM32_MSI_ENABLED'
is redundant but intentional as it is in line with remaining part of the
function (HSE/HSI cases).

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