pwm: stm32: Calculate prescaler with a division instead of a loop

Instead of looping over increasing values for the prescaler and testing
if it's big enough, calculate the value using a single division.

Link: https://lore.kernel.org/r/498a44b313a6c0a84ccddd03cd67aadaaaf7daf2.1710711976.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>