dmaengine: xilinx: drop redundant spin_lock due to merge issue

When merging the upstream commit 8bf57418 ("dma: xilinx_dpdma: Fix
locking"), a spin_lock function should be dropped, or else there will be
continous spin_lock calling and this will result in lockup.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>