dmaengine: idma64: convert to __ffs()

We replace __fls() by __ffs() since we have to find a *minimum* data width that
satisfies both source and destination.

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