dmaengine.h: remove redundant else keyword

dmaengine_device_control returns -ENOSYS in case the dma driver doesn't have
such functionality.

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>