pwm: stm32: Make use of mul_u64_u64_div_u64_roundup()

pwm: stm32: Make use of mul_u64_u64_div_u64_roundup()

When the driver was converted to the waveform API the need for this
function arised but at that time this function didn't exist yet. In the
meantime it's available, so switch to the global function and drop the
driver specific implementation.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/788319f0fff963feca4df3c5fcdd471dcf70ccdf.1776264104.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>