dmaengine: bcm-sba-raid: Remove reqs_free_count from sba_device

The reqs_free_count member of sba_device is not used anywhere
hence no point in tracking number of free sba_request.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>