dmaengine: imx-dma: fix missing unlock on error in imxdma_xfer_desc()

Add the missing unlock on the error handling path in function
imxdma_xfer_desc().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>