simplify oneshot mode selection; use zero PWM rate as indicator

src/drivers/px4fmu/fmu.cpp

src/drivers/stm32/drv_io_timer.c

src/drivers/stm32/drv_io_timer.h

src/drivers/stm32/drv_pwm_servo.c

src/modules/px4iofirmware/mixer.cpp

src/modules/px4iofirmware/px4io.c

src/modules/px4iofirmware/registers.c

src/systemcmds/pwm/pwm.c