libefi: Tag an unreachable switch default.

Coverity reports an uninitialized "len" 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:	1347796

sys/boot/efi/libefi/efi_console.c

@@ -266,6 +266,8 @@ CL(int direction)
 	case 2:         /* entire line */
 		len = x;
 		break;
+	default:	/* NOTREACHED */
+		__unreachable();
 	}
 
 	if (cury == y - 1)