STM32F30x - Working parallel PWM. Timer IRQ handle was not defined for
TIM8 correctly. Software serial conficted with parallel PWM input. Fixed
crash due to incorrect timerConfig index calculation code.

The crash occurred when capture compare interrupt handlers attempted to
call an invalid callback.

Note: the crash does not affect existing baseflight code since the
formula is OK when using a 2 dimensional array with equal dimensions.

Formula details here:
https://docs.google.com/spreadsheets/d/1UuxnC0VKiprvt1wt3KXffefEWJUb_-ILFLQuP377Anw