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

commit 720dfd250e48a8c7fd1b2b8645955413989c4ee0 upstream.

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>