hw/dma/pl080: Remove hw_error() if DMA is enabled

hw/dma/pl080: Remove hw_error() if DMA is enabled

The PL08x model currently will unconditionally call hw_error()
if the DMA engine is enabled by the guest. This has been
present since the PL080 model was edded in 2006, and is
presumably either unintentional debug code left enabled,
or a guard against untested DMA engine code being used.

Remove the hw_error(), since we now have a guest which
will actually try to use the DMA engine (the self-test
binary for the AN505 MPS2 FPGA image).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>