DMA: PL330: Fix potential NULL pointer dereference in pl330_submit_req()

'r->cfg' is being checked for NULL. However, it is dereferenced
in the previous statements. Thus moving those statements within
the check.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>