dw_dmac: introduce dwc_dump_chan_regs to dump registers

There is three places where values of the most significant registers were
printed. Make such piece of code as separate function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.linux@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>