rsp_mixer: fix condition of DMA end
rsp_mixer: fix condition of DMA end

This is just a very slight optimization: we can use the unrolled loop
also in the case in which pos+step*8 reaches exactly DMA end. Before,
there was an off-by-one error which fell back to the normal loop in
this edge case.

DragonMinded/libdragon,sp1187/libdragon,DragonMinded/libdragon,sp1187/libdragon,sp1187/libdragon,DragonMinded/libdragon