dac_write (used by AnalogOut.write) calls HAL_DAC_Start
every time. It is required to call HAL_DAC_Start only once.
HAL_DAC_Start uses internally HAL_Delay(1) making AnalogOut
not suitable for use in high speed application.
This change removes call to HAL_DAC_Start in dac_write and
moves it to analogout_init.