solo_p2m_dma 4-byte writes can silently fail

[curtishall]

4-byte writes done with solo_p2m_dma can appear to succeed, but not actually write to memory. Some reads on 4 or 16 byte boundaries may exhibit the same problem. See https://github.com/bluecherrydvr/solo6x10/blob/32a109c8/solo6010-disp.c#L182 (solo_set_motion_block @ 32a109c) for the problematic code, which was detected by reading back the same location immediately after writing and comparing values.