dmaengine: xgene-dma: Fix overwritting DMA tx ring

This patch fixes an over flow issue with the TX ring descriptor. Each
descriptor is 32B in size and an operation requires 2 of these
descriptors.

Signed-off-by: Rameshwar Prasad Sahu <rsahu@apm.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>