dmaengine: bcm-sba-raid: Reduce locking context in sba_alloc_request()

We don't require to hold "sba->reqs_lock" for long-time
in sba_alloc_request() because lock protection is not
required when initializing members of "struct sba_request".

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