dmaengine: xilinx_dma: Fix kernel-doc warnings

dmaengine: xilinx_dma: Fix kernel-doc warnings

Modify the prototype from xilinx_dma_tx_descriptor to
xilinx_dma_alloc_tx_descriptor and xilinx_dma_channel_set_config
to xilinx_vdma_channel_set_config in API description to
fix below linux kernel-doc warnings.

drivers/dma/xilinx/xilinx_dma.c:800: warning: expecting
prototype for xilinx_dma_tx_descriptor(). Prototype was
for xilinx_dma_alloc_tx_descriptor() instead.

drivers/dma/xilinx/xilinx_dma.c:2471: warning: expecting
prototype for xilinx_dma_channel_set_config(). Prototype
was for xilinx_vdma_channel_set_config() instead.

Signed-off-by: Shravya Kumbham <shravya.kumbham@xilinx.com>
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Link: https://lore.kernel.org/r/1631525316-2323-1-git-send-email-radhey.shyam.pandey@xilinx.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>