SPI_DEVICE_NO_DUMMY macro

There are timing issue when reading at high frequency (the frequency is related to whether iomux pins are used, valid time after slave sees the clock). - In half-duplex mode, the driver automatically inserts dummy bits before reading phase to fix the timing issue. Set this flag to disable this feature. - In full-duplex mode, however, the hardware cannot use dummy bits, so there is no way to prevent data being read from getting corrupted. Set this flag to confirm that you're going to work with output only, or read without dummy bits at your own risk.