dmaengine: xilinx: dpdma: Make symbol 'dpdma_debugfs_reqs' static

The sparse tool complains as follows:

drivers/dma/xilinx/xilinx_dpdma.c:349:37: warning:
 symbol 'dpdma_debugfs_reqs' was not declared. Should it be static?

This variable is not used outside of xilinx_dpdma.c, so this commit
marks it static.

Fixes: 1d220435 ("dmaengine: xilinx: dpdma: Add debugfs support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20200818112217.43816-1-weiyongjun1@huawei.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>