dmaengine: bcm-scm-raid: remove redundant null check on req

Req is never null on at the point of the null check, so
remove this redundant check and just return &req->tx.

Detected by CoverityScan, CID#1436147 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>