drm/nouveau/clk: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/gpu/drm/nouveau/nvkm/subdev/clk/nv50.c

@@ -313,6 +313,7 @@ nv50_clk_read(struct nvkm_clk *base, enum nv_clk_src src)
 		default:
 			break;
 		}
+		break;
 	default:
 		break;
 	}