hw/dma/pl080: Correct bug in register address decode logic

hw/dma/pl080: Correct bug in register address decode logic

A bug in the handling of the register address decode logic
for the PL08x meant that we were incorrectly treating
accesses to the DMA channel registers (DMACCxSrcAddr,
DMACCxDestaddr, DMACCxLLI, DMACCxControl, DMACCxConfiguration)
as bad offsets. Fix this long-standing bug.

Fixes: https://bugs.launchpad.net/qemu/+bug/1637974
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>