[media] tw686x: be explicit about the possible dma_mode options

Users won't know what to put in this module option if it isn't
described.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/pci/tw686x/tw686x-core.c

@@ -91,7 +91,7 @@ static int tw686x_dma_mode_set(const char *val, struct kernel_param *kp)
 }
 module_param_call(dma_mode, tw686x_dma_mode_set, tw686x_dma_mode_get,
 		  &dma_mode, S_IRUGO|S_IWUSR);
-MODULE_PARM_DESC(dma_mode, "DMA operation mode");
+MODULE_PARM_DESC(dma_mode, "DMA operation mode (memcpy/contig/sg, default=memcpy)");
 
 void tw686x_disable_channel(struct tw686x_dev *dev, unsigned int channel)
 {