DMA: shdma: tidyup callback chunk finder

Current shdma is using "last" which indicates last desc which needs to have
callback function. But that desc's chunks is always 1, we can use it as finder

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
[reflown changelog for readablity]
Signed-off-by: Vinod Koul <vinod.koul@intel.com>