drivers/clock: fix stm32 flash latency calculation in init function

This commit fixes the flash latency calulation in clock_control_init
for stm32wb and stm32wl series:
For these series new_hclk_freq can't be used to set the flash latency,
because the flash clk uses a different prescaler.

Without this fix, the flash latency could be set to an inadequat value
in cases wehere the new AHB3/AHB4 prescaler is different from the
new cpu1 prescler.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>