dmaengine: idma64: use local variable to index descriptor

Since a local variable contains the number of hardware desriptors at the
beginning of idma64_desc_fill() we may use it to index the last descriptor as
well.

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