clock_control: stm32f1: fix STM32_PLL_XTPRE check

This is always defined since:

755d09e1 include/drivers/clock_control: stm32: Update for STM32F1
support

So the condition has currently no effect and causes the prescaler to
always be set to /2.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>