dw_dmac: update tx_node_active in dwc_do_single_block

The "else" keyword in the dw_dma_tasklet is removed as well. All together
simplifies the logic of the code and understanding of what is happening there.

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