dmaengine: xilinx_dma: Drop SG support for VDMA IP

xilinx_vdma_start_transfer() is used only for VDMA IP, still it contains
conditional code on has_sg variable. has_sg is set only whenever the HW
does support SG mode, that is never true for VDMA IP.

This patch drops the never-taken branches.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>