dmaengine: xgene-dma: Fix the lock to allow client for further submission of requests

This patch provides the fix in the cleanup routing such that client can perform
further submission by releasing the lock before calling client's callback function.

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