dw_dmac: move soft LLP code from tasklet to dwc_scan_descriptors

The proper place for the main logic of the soft LLP mode is
dwc_scan_descriptors. It prevents to get the transfer unexpectedly aborted in
case the user calls dwc_tx_status.

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>