hw/dma/i8257: Use qemu_log_mask(UNIMP) instead of fprintf

hw/dma/i8257: Use qemu_log_mask(UNIMP) instead of fprintf

Avoid to clutter stdout until explicitly requested (with -d unimp):

  $ qemu-system-mips64el -M fulong2e -bios pmon_2e.bin
  dma: command df not supported
  dma: command df not supported
  dma: command df not supported
  dma: command df not supported

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190212145322.30974-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>