dmaengine: dw: introduce dwc_dostart_first_queued() helper

commit e7637c6c upstream.

We have a duplicate code which starts first descriptor in the queue. Let's make
this as a separate helper that can be used in future as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Cc: "Petallo, MauriceX R" <mauricex.r.petallo@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>