dmaengine: virt-dma: add missing MODULE_DESCRIPTION() macro

With ARCH=loongarch, make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/dma/virt-dma.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240616-md-loongarch-drivers-dma-virt-dma-v1-1-70ed3dcbf8aa@quicinc.com


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