dmaengine: dw: convert to __ffs()

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

While here, rename dwc_fast_fls() to dwc_fast_ffs() which it really is.

Fixes: 4c2d56c5 (dw_dmac: introduce dwc_fast_fls())
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>