Fix channel selection for DMA2 Stream6.

[manuelilg]

For TIM1_CH3 the wrong Channel gets selected during timer definition/configuration. Correct is to use Channel6 which is only for TIM1_CH3 instead of the Channel0 which is for TIM1_CH1, TIM1_CH2 and TIM1_CH3. See also https://github.com/iNavFlight/inav/blob/release_7.1.1/src/main/drivers/timer_def_stm32f4xx.h#L35 and table below

At start up, when only the DSHOT command 0 is send, everything is ok. But as soon as other commands/values are send then this happens:

Additional Infos: Tested on Target: FLYWOOF411 But the other controllers (F405 and F722) have the same DMA request mapping. ESC protocol: DSHOT300