Fix and restore PR: #9343 Fix delays and rounding in samd PWM #9448
Description
The code changes in #9343 were reverted by #9447, because they broke screen backlighting on SAMD51 boards like PyPortal (and maybe other PWM things as well), as reported in #9443.
The intention of #9343 is good, but it needs to be fixed.
Note that #9343 did not affect backlighting on Hallowing M0, so I'm guessing the #9343 changes were OK for SAMD21 but not SAMD51.
@timchinowsky I hope you are able to take a look at this. Thanks.