dmaengine: mv_xor: Rename function for consistent naming

The current function names isn't very consistent, and functions with the
same prefix might operate on either a channel or a descriptor, which is
kind of confusing.

Rename these functions to have a consistent and clearer naming scheme.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>