The assumption that all STM32 timers have exactly four channels no
longer holds. E.g. the STM32L4 has the following general purpose timers:
- TIM2: 32 bit, 4 channels
- TIM15: 16 bit, 2 channels
- TIM16: 16 bit, 1 channel
Hence, a new field is added to the timer configuration to also contain
the number of timer channels. Due to alignment the `struct` previously
was padded by 16 bit, so adding another 8 bit field doesn't increase
its size.
For backward compatibility, a value of `0` is considered as alias for
`TIMER_CHANNEL_NUMOF` (or 4), so that the number of timer channels
only needs to be set when the timer is different from the typical 4
channel timer. This helps backward compatibility.
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