Fix stereo channels being reversed
By default RISC OS/SoundDMA generates 32bit sample frames where the right channel is in the low 16 bits and the left channel is in the high 16 bits, as required by VIDC's 'wide' sound FIFO which stores a full 32bit frame per entry. This is in contrast to most newer systems RISC OS supports, with 'narrow' FIFOs which store only the data for a single channel per entry. Since I2S expects the left channel to be transmitted first, this generally means that RISC OS must switch to generating data such that the left channel is in the low 16 bits of each word (as supported via the audio HAL device 'StereoReverse' flag), or, as in this case, by instructing the hardware to swap the channels for us.