dmaengine: bcm-sba-raid: serialize dma_cookie_complete() using reqs_lock

As-per documentation in driver/dma/dmaengine.h, the
dma_cookie_complete() API should be called with lock
held.

This patch ensures that Broadcom SBA RAID driver calls
the dma_cookie_complete() API with reqs_lock held.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>