[stm32] fix spektrum on STM32F4

on F4 TIM6 is not running with AHB_CLK.
Also replace the custom DelayUs using TIM6 directly with sys_time_usleep.
[stm32] spektrum: TIM6 can only count up
[stm32] uart rx and config pin for spektrum can be separated

this is needed on Apogee boards
[stm32] radio_control spektrum polarity for apogee