dmaengine: hsu: remove excessive lock

All hardware accesses are done under virtual channel lock. That's why specific
channel lock is excessive and can be removed safely. This has been tested on
Intel Medfield and Merrifield.

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