drivers/can: stm32fd fix not applied clock divider

The configurable CAN clock divider CAN_STM32FD_CLOCK_DIVISOR
was not applied during initialization, because write protection
was not disabled.

While the clock divider was not applied, it was still used in clock rate
calculation, therefore resulting in incorrect bus speed setup.

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