dev/ow: Tag an unreachable switch default.

Coverity reports an uninitialized "dir" in case the switch defaults
without hitting any case. Respect the original intent and quell the
false positive with the relatively new __unreachable() builtin.

CID:	1331566

sys/dev/ow/ow.c

@@ -401,6 +401,8 @@ again:
 				if (++retries > 5)
 					return (EIO);
 				goto again;
+			default: /* NOTREACHED */
+				__unreachable();
 			}
 			if (dir) {
 				OWLL_WRITE_ONE(lldev, &timing_regular);